Transmitting and receiving method, and radio apparatus utilizing the same

ABSTRACT

A processing unit transmits, from at least one of a plurality of antennas, data corresponding respectively to the plurality of antennas. A control unit generates request signals with which to let a second radio apparatus supply information on rates at the second radio apparatus. When transmitting the request signal, the processing unit also transmits, from a plurality of antennas which includes antennas other than the antennas that transmit the data, known signals corresponding respectively to the plurality of antennas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to transmitting and receivingtechnologies, and it particularly relates to a transmitting andreceiving method, in which signals are transmitted from a plurality ofantennas and the signals are received by a plurality of antennas, and aradio apparatus utilizing said method.

2. Description of the Related Art

An OFDM (Orthogonal Frequency Division Multiplexing) modulation schemeis one of multicarrier communication schemes that can realize thehigh-speed data transmission and are robust in the multipathenvironment. This OFDM modulation scheme has been used in the wirelessstandards such as IEEE802.11a/g and HIPERLAN/2. The burst signals insuch a wireless LAN are generally transmitted via a time-varying channelenvironment and are also subject to the effect of frequency selectivefading. Hence, a receiving apparatus generally carries out the channelestimation dynamically.

In order for the receiving apparatus to carry out the channelestimation, two kinds of known signals are provided within a burstsignal. One is the known signal, provided for all carries in thebeginning of the burst signal, which is the so-called preamble ortraining signal. The other one is the known signal, provided for part ofcarriers in the data area of the burst signal, which is the so-calledpilot signal (See Reference (1) in the following Related Art List, forinstance).

2. Related Art List

-   (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel    Estimation Techniques Based on Pilot Arrangement in OFDM Systems”,    IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229,    September 2002.

In wireless communications, adaptive array antenna technology is one ofthe technologies to realize the effective utilization of frequencyresources. In adaptive array antenna technology, the directionalpatterns of antennas are controlled by controlling the amplitude andphase of signals, to be processed, in a plurality of antennas,respectively. In adaptive array antenna technology, the amplitude andphase of signals transmitted from and received by a plurality ofantennas, respectively, are so controlled as to form a directionalpattern of the antenna. One of techniques to realize higher datatransmission rates by using such an adaptive array antenna technology isthe MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, atransmitting apparatus and a receiving apparatus are each equipped witha plurality of antennas, and a channel corresponding to each of theplurality of antennas is set. That is, channels up to the maximum numberof antennas are set for the communications between the transmittingapparatus and the receiving apparatus so as to improve the datatransmission rates. Moreover, combining this MIMO system with atechnique such as the OFDM modulation scheme results in a higher datatransmission rate.

In the MIMO system, the data rate can also be adjusted by increasing thenumber of antennas to be used for data communications. Furthermore, thedata rate can be adjusted in greater detail by applying the adaptivemodulation to the MIMO system. To perform such an adjustment of datarates more reliably it is desired that the transmitting apparatusalready acquire from the receiving apparatus the information on datarates suited for the radio channel between the transmitting apparatusand the receiving apparatus (hereinafter referred to as “rateinformation”). If, on the other hand, the rate information is nottransmitted periodically in the MIMO system, the transmitting apparatustransmits to the receiving apparatus a signal by which to request thetransmission of the rate information (hereinafter referred to as “raterequest signal”).

Examples of the combinations of directivity patterns in the transmittingapparatus and receiving apparatus in a MIMO system are as follows. Oneexample is a case where the antennas of a transmitting apparatus haveomni patterns and the antennas of a receiving apparatus have patterns inadaptive array signal processing. Another example is a case where boththe antennas of the transmitting apparatus and those of the receivingapparatus have patterns in adaptive array signal processing. The systemcan be simplified in the former case. In the latter case, however, thedirectivity patterns of antennas can be controlled in greater detail, sothat the characteristics thereof can be improved. Since in the lattercase the transmitting apparatus performs adaptive array signalprocessing for transmission, it is necessary to receive beforehand fromthe receiving apparatus the known signals by which to estimate channels.To improve the accuracy of controlling the adaptive array antennas, itis desirable that the transmitting apparatus acquire the respectivechannel characteristics between a plurality of antennas contained in thetransmitting apparatus and those in the receiving apparatus. For thisreason, the receiving apparatus transmits from all of antennas the knownsignals for channel estimation. In this patent specification, the knownsignals, for use with channel estimation, transmitted from a pluralityof antennas will be called “training signals” independently of thenumber of antennas to be used for data communication.

Under these circumstances, the inventors of the present invention cameto recognize the following problems to be solved. If any error iscontained in the rate information determined by the receiving apparatus,an error will be caused in communications by a MIMO system and thereforethe transmission quality and effective data rate will deteriorate. Thus,the determination of rate information by the receiving apparatus needsto be done with accuracy. In order to raise the effective data rate, itis desired that the transmission of signals other than the data, forexample, the rate request signal or training signals, be minimized. Whenthe transmitting apparatus or the receiving apparatus is powered by abattery, the lower power consumption is desired.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoingcircumstances and an object thereof is to provide a receiving method bywhich to improve control accuracy in transmitting data and to providealso a radio apparatus utilizing said method.

In order to solve the above problems, a radio apparatus according to apreferred embodiment of the present invention comprises: a transmitterwhich transmits data corresponding respectively to a plurality ofantennas from at least one of the plurality of antennas to a targetedradio apparatus compatible with a variable data rate; and a control unitwhich generates a request signal by which to inform said radio apparatusof information on a data rate suitable for a radio channel between saidradio apparatus and the targeted radio apparatus and which transmits thegenerated request signal as the data from the transmitter. Whentransmitting the request signal, the transmitter also transmits, from aplurality of antennas that include antennas other than the at least oneof antennas to transmit the data, known signals correspondingrespectively to the plurality of antennas.

The factors to determine the “data rate” are, for example, modulationschemes, error correction coding rates and the number of antennas usedin a MIMO system. Here, the “data rate” may be determined by arbitrarycombination of these and other factors or it may be determined by one ofthese and other factors.

According to this embodiment, the known signals are transmitted from aplurality of antennas when the request signal is sent to a radioapparatus to be communicated with. As a result thereof, information, ondata rates, generated anew based on the known signals can be acquired,thus contributing to improving the information accuracy.

When transmitting the request signal, the transmitter performsbeamforming on at least the known signals corresponding respectively tothe plurality of antennas. In this case, the signal strength in atargeted radio apparatus can be raised by performing the beamforming, sothat information on data rates having faster values can be obtained.

The radio apparatus may further comprise a selector which selects, fromamong the plurality of antennas, at least one antenna to be used whenreceiving the data from the targeted radio apparatus. The transmittermay transmit the known signals from the antenna selected by theselector. In this case, the number of antennas that should transmitcontrol signals can be reduced, so that the power consumption can bereduced.

The radio apparatus may further comprise a receiver which receives, by aplurality of antennas, known signals for use with receiving from thetargeted radio apparatus. The selector may derive radio qualitiescorresponding respectively to the plurality of antennas, based on theknown signals received by the receiver, and may select preferentially anantenna whose radio quality is satisfactory.

The “radio quality” is the quality of a radio link and it may beevaluated by arbitrary parameters that include signal strength, delayspread and interference amount. The radio quality may be evaluated bythe combination thereof. In such a case, since an antenna whose radioquality is desirable is selected preferentially, the deterioration ofquality of data transmission can be prevented.

The radio apparatus may further comprise: a receiver which receives, bya plurality of antennas, known signals for use with receiving from thetargeted radio apparatus; and a selector which selects, from among theplurality of antennas, at least one antenna that should transmit theknown signals. The selector may derive radio qualities correspondingrespectively to the plurality of antennas, based on the known signalsreceived by the receiver, and may select preferentially an antenna whoseradio quality is satisfactory.

Another preferred embodiment according to the present invention relatesalso to a radio apparatus. This apparatus comprises: a selector whichselects, among a plurality of antennas, at least one antenna to be usedwhen data are received from a targeted radio apparatus; and atransmitter which transmits data corresponding to each antenna, from anantenna contained in the at least one antenna selected by the selectorto the targeted radio apparatus and which transmits also a trainingsignal corresponding to each antenna, from the at least one antennaselected by the selector.

According to this embodiment, the known signals are transmitted from theantennas that should transmit data. As a result, the deterioration ofdirectivity in a targeted radio apparatus can be prevented. Moreover,since the antennas that should receive the data are selected, so thatthe power consumption can be reduced.

Still another preferred embodiment according to the present inventionrelates also to radio apparatus. This apparatus is a radio apparatusthat receives variable-rate data, transmitted from at least one of aplurality of antennas, which corresponds to each antenna, and the radioapparatus comprises: a receiver which receives known signals,transmitted from a plurality of antennas containing also antennas otherthan the at least one antenna to receive the data, which correspondrespectively to the plurality of antennas; a receiving response vectorcomputing unit which computes receiving response vectors correspondingrespectively to the plurality of antennas, based on the known signalsreceived by the receiver; a correlation computing unit which computescorrelation among the receiving response vectors correspondingrespectively to the plurality of antennas, from the receiving responsevectors computed by the receiving response vector computing unit; and adetermining unit which determines a data rate for data, based on thecorrelation computed by the correlation computing unit.

According to this embodiment, the correlation among the receivingresponse vectors are taken into account. Thus, the effects of amongsignals transmitted respectively from a plurality of antennas can bereflected and the degree of accuracy in data rate thus determined can beimproved.

Still another preferred embodiment according to the present inventionrelates also to radio apparatus. This apparatus is a radio apparatusthat receives variable-rate data, transmitted from at least one of aplurality of antennas, which corresponds to each antenna, and the radioapparatus comprises: a receiver which receives known signals,transmitted from a plurality of antennas containing also antennas otherthan the at least one antenna to receive the data, which correspondrespectively to the plurality of antennas; a receiving response vectorcomputing unit which computes receiving response vectors correspondingrespectively to the plurality of antennas, based on the known signalsreceived by the receiver; a power ratio computing unit which computespower ratios among the receiving response vectors correspondingrespectively to the plurality of antennas, from the receiving responsevectors computed by the receiving response vector computing unit; and adetermining unit which determines a data rate for data, based on thepower ratios computed by the power ratio computing unit.

According to this embodiment, the ratios of strength among the receivingresponse vectors are taken into account. Thus, the effects of amongsignals transmitted respectively from a plurality of antennas can bereflected and the degree of accuracy in data rate thus determined can beimproved.

The known signal received by the receiver uses a plurality of carriers,and the determining unit may determine a data rate for data, based on astate of any of the plurality of carriers. “Any of the plurality ofcarriers” may be a carrier whose correlation or ratio of strength forall carriers is most desirable or undesirable, or a carrier thatcomplies with a predetermined rule. The average of correlation or ratiosof strength for all carrier may be calculated so as to be correspondedto a pseudo carrier. Also, the average or ratios of strength for part ofcarriers may be calculated so as to be corresponded to a pseudo carrier.In this case, the present invention can be applied to a system using aplurality of carriers. The “state” includes correlation or power ratios,and may be information indicative of the quality of a signal.

The receiver may receive also a request for information on the data rateat the time of receiving the known signals, and the apparatus mayfurther comprise a notifying unit which conveys the data rate determinedby the determining unit, as a response to the request received by thereceiver. In this case, when the known signals are received, the requestsignal is also received. As a result, the information on data rates thusdetermined can be notified and the highly accurate data rates can besupplied.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: a generatorwhich generates a burst signal that contains: first known signalscorresponding respectively to at least one of a plurality of antennas;second known signals corresponding respectively to a plurality ofantennas containing also antennas other than the at least one antenna totransmit the first known signals; and data corresponding respectively tothe at least one antenna to transmit the first known signals; and atransmitter which transmits the burst signal generated by the generator,via the plurality of antennas.

One example of the “first known signal” is a signal by which to set AGCin a targeted radio apparatus. One example of the “second known signal”is a signal by which to estimate channel characteristics in a targetedradio apparatus. According to this embodiment, a structure is such thatan antenna to transmit the first known signals is the same as that totransmit the data. Thus, the estimation result by the first knownsignals at a receiving side can be used for the receiving of data andtherefore the characteristics of data receiving can be improved.

Among the second known signals the generator may assign, at differenttimings, a portion corresponding to the at least one antenna to transmitthe first known signals and a portion corresponding to the antennasother than the at least one antenna to transmit the first known signals.In this case, among the second known signals, the effect of the portioncorresponding to the antennas other than the at least one antenna totransmit the first known signals on the portion corresponding to the atleast one antenna to transmit the first known signals can be reduced.Thus, the accuracy of estimation, at a receiving side, based on thesecond known signals at a portion corresponding to the at least oneantenna to transmit the first known signals can be improved.

The generator may increase the number of antennas that should transmitthe first known signals up to the number of antennas that shouldtransmit the second known signals, segment data correspondingrespectively to antennas prior to increasing the number thereof, andassociate the segmented data to antennas whose number has beenincreased. In this case, a structure is such that an antenna to transmitthe first known signals is the same as that to transmit the data. Thus,the estimation result by the first known signals at a receiving side canbe used for the receiving of data and therefore the characteristics ofdata receiving can be improved.

While using a plurality of subcarriers, the generator may generate thedata contained in the burst signal and segment the data on asubcarrier-by-subcarrier basis. In this case, interference among thesegmented data can be reduced.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: atransmitter which transmits burst signals respectively from a pluralityof antennas; a generator which generates a burst signal, to betransmitted from the transmitter, that contains known signalscorresponding respectively to the plurality of antennas and dataassigned posterior to the known signals; and a determining unit whichdetermines a data rate of data contained in the burst signal generatedby the generator. When the data corresponds to at least one of theplurality of antennas, the generator associates said data to theplurality of antennas by increasing the number of antennas to beassociated thereto, and when the generator associates said data to theplurality of antennas, the determining unit determines that the datarate is lower than that prior to associating the data to the pluralityof antennas.

According to this embodiment, even if data are associated respectivelyto a plurality of antennas and the radio channel characteristics fromthe thus associated antennas are not suited to the data, the occurrenceof data error can be reduced by lowering the data rate.

While using a plurality of subcarriers for the known signals and data,the generator varies a combination of subcarriers to be usedrespectively for the known signals, for each of the plurality ofantennas, and when the data are associated to the plurality of antennas,a combination of subcarriers in the known signals transmitted from thesame antenna as the data may be used for said data. In this case, thesame subcarries are used for the known signals and data corresponding toone antenna. Thereby the selection of subcarriers to be used for therespective data can be facilitated.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: atransmitter which transmits burst signals respectively from a pluralityof antennas; and a generator which generates a burst signal, to betransmitted from the transmitter, that contains known signalscorresponding respectively to the plurality of antennas and dataassigned posterior to the known signals. The generator includes: a firstmeans for associating the data to antennas that should transmit theknown signals, if the data corresponds to at least one of the pluralityof antennas, by increasing the number of antennas to be associatedthereto; and a second means for varying a combination of subcarriers tobe used respectively for the known signals, for each of the plurality ofantennas, while using a plurality of subcarriers for the known signalsand data, and for using a combination of subcarriers in the known signaltransmitted from the same antenna as the data, for said data, when thedata are associated to a plurality of antennas.

According to this embodiment, if data are associated to a plurality ofantennas, the same subcarries are used for the known signals and datacorresponding to one antenna. Thereby the selection of subcarriers to beused for the respective data can be facilitated.

Still another preferred embodiment according to the present inventionrelates to a transmitting method. This is a method for transmitting datacorresponding respectively to a plurality of antennas from at least oneof the plurality of antennas to a targeted radio apparatus compatiblewith a variable data rate, and the method is characterized in that arequest signal by which to inform the radio apparatus of information ona data rate suitable for a radio channel between the radio apparatus andthe targeted radio apparatus is generated and when the generated requestsignal is transmitted as the data, known signals correspondingrespectively to a plurality of antennas are also transmitted from theplurality of antennas that include antennas other than the at least oneof antennas to transmit the data.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This is a method for transmittingdata corresponding respectively to a plurality of antennas from at leastone of the plurality of antennas to a targeted radio apparatus, and themethod is characterized in that at least one antenna to be used whendata are received from a targeted radio apparatus is selected from amonga plurality of antennas and a known signal corresponding to each antennais also transmitted from the selected at least one antenna.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises:transmitting data corresponding respectively to a plurality of antennasfrom at least one of the plurality of antennas to a targeted radioapparatus compatible with a variable data rate; and generating a requestsignal by which to inform a radio apparatus of information on a datarate suitable for a radio channel between the radio apparatus and thetargeted radio apparatus. When transmitting the generated request signalas the data, the transmitting is such that known signals correspondingrespectively to a plurality of antennas are also transmitted from aplurality of antennas that include antennas other than the at least oneof antennas to transmit the data.

When transmitting the request signal, the transmitting may be such thatbeamforming is performed on at least the known signals correspondingrespectively to the plurality of antennas. The method may furthercomprise selecting, from among the plurality of antennas, at least oneantenna to be used when receiving the data from the targeted radioapparatus, wherein the transmitting may be such that the known signalsare transmitted from the selected antenna. The method may furthercomprise a receiving, by a plurality of antennas, known signals for usewith receiving from the targeted radio apparatus, wherein the selectingmay be such that radio qualities corresponding respectively to theplurality of antennas are derived based on the received known signalsand an antenna whose radio quality is satisfactory is selectedpreferentially.

The method may further comprise: receiving, by a plurality of antennas,known signals for use with receiving from the targeted radio apparatus;and selecting, from among the plurality of antennas, at least oneantenna that should transmit the known signals. The selecting may besuch that radio qualities corresponding respectively to the plurality ofantennas are derived based on the received known signals and an antennawhose radio quality is satisfactory is selected preferentially.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises: selecting,among a plurality of antennas, at least one antenna to be used when dataare received from a targeted radio apparatus; and transmitting datacorresponding to each antenna, from an antenna contained in the at leastone antenna selected by the selecting to the targeted radio apparatusand transmitting also a training signal corresponding to each antenna,from the at least one antenna selected by the selecting.

The method may further comprise: generating a burst signal, to betransmitted in the transmitting, that contains known signals and data;and determining a data rate of data contained in the burst signalgenerated in the generating, wherein when the data is associated to atleast one of antennas that should transmit the known signal, thegenerating may be such that said data are associated to the antennasthat should transmit the known signal by increasing the number ofantennas to be associated thereto, and wherein when, in the generating,data are associated to the antennas that should transmit the knownsignals, the determining may be such that a data rate, for the antennathat should transmit the known signals, is determined to be lower thanthat before the data are associated thereto. While using a plurality ofsubcarriers for the known signals and data, the generating may be suchthat a combination of subcarriers to be used respectively for the knownsignals are varied for each of the plurality of antennas, and when thedata are associated to the antennas that should transmit the knownsignals, a combination of subcarriers in the known signals transmittedfrom the same antenna as the data is used for said data.

The method may further comprise generating burst signals, to betransmitted in the transmitting, which contain known signals and data,wherein the generating includes: associating said data to antennas thatshould transmit the known signals, if the data corresponds to at leastone of antennas that should transmit the known signals, by increasingthe number of antennas to be associated thereto; and varying acombination of subcarriers to be used respectively for the knownsignals, for each of the plurality of antennas, while using a pluralityof subcarriers for the known signals and data, and using a combinationof subcarriers in the known signal transmitted from the same antenna asthe data, for said data, when the data are associated to the antennasthat should transmit the known signals.

Still another preferred embodiment according to the present inventionrelates to a receiving method. This is a method for receivingvariable-rate data, transmitted from at least one of a plurality ofantennas, which corresponds to each antenna, and the method ischaracterized in that, based on known signals, transmitted from aplurality of antennas containing also antennas other than the at leastone antenna to receive the data, which correspond respectively to theplurality of antennas, receiving response vectors correspondingrespectively to the plurality of antennas are computed, correlationamong the receiving response vectors corresponding respectively to theplurality of antennas are computed from the computed receiving responsevectors, and a data rate for data is determined based on thecorrelation.

Still another preferred embodiment according to the present inventionrelates also to a receiving method. This is a method for receivingvariable-rate data, transmitted from at least one of a plurality ofantennas, which corresponds to each antenna, and the method ischaracterized in that, based on known signals, transmitted from aplurality of antennas containing also antennas other than the at leastone antenna to receive the data, which correspond respectively to theplurality of antennas, receiving response vectors correspondingrespectively to the plurality of antennas are computed, power ratiosamong the receiving response vectors corresponding respectively to theplurality of antennas are computed from the computed receiving responsevectors, and a data rate for data is determined based on the powerratios.

Still another preferred embodiment according to the present inventionrelates to a receiving method. This is a method for receivingvariable-rate data, transmitted from at least one of a plurality ofantennas, which corresponds to each antenna, and the method comprises:receiving known signals, transmitted from a plurality of antennascontaining also antennas other than the at least one antenna to receivethe data, which correspond respectively to the plurality of antennas;computing receiving response vectors corresponding respectively to theplurality of antennas, based on the received known signals; computingcorrelation among the receiving response vectors correspondingrespectively to the plurality of antennas from the computed receivingresponse vectors; and determining a data rate for data based on thecomputed correlation.

Still another preferred embodiment according to the present inventionrelates also to a receiving method. This is a method for receivingvariable-rate data, transmitted from at least one of a plurality ofantennas, which corresponds to each antenna, and the method comprises:receiving known signals, transmitted from a plurality of antennascontaining also antennas other than the at least one antenna to receivethe data, which correspond respectively to the plurality of antennas;computing receiving response vectors corresponding respectively to theplurality of antennas, based on the received known signals; computingpower ratios among the receiving response vectors correspondingrespectively to the plurality of antennas from the computed receivingresponse vectors; and determining a data rate for data, based on thecomputed power ratios.

The known signal received in the receiving uses a plurality of carriersand the determining may be such that a data rate for data is determinedbased on a state of any of the plurality of carriers. The receiving maybe such that, a request for information on the data rate is alsoreceived at the time of receiving the known signals and the method mayfurther comprise notifying the data rate determined in the determining,as a response to the received request.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method includes transmittinga burst signal that contains: first known signals correspondingrespectively to at least one of a plurality of antennas; second knownsignals corresponding respectively to a plurality of antennas containingalso antennas other than the at least one antenna to transmit the firstknown signals; and data corresponding respectively to the at least oneantenna to transmit the first known signals.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises: generatinga burst signal that contains first known signals correspondingrespectively to at least one of a plurality of antennas, second knownsignals corresponding respectively to a plurality of antennas containingalso antennas other than the at least one antenna to transmit the firstknown signals and data corresponding respectively to the at least oneantenna to transmit the first known signals; and transmitting the burstsignal generated in the generating, via the plurality of antennas.

The generating may be such that, among the second known signals, aportion corresponding to the at least one antenna to transmit the firstknown signals and a portion corresponding to the antennas other than theat least one antenna to transmit the first known signals are assigned atdifferent timings. The generating may be such that the number ofantennas that should transmit the first known signals are increased upto the number of antennas that should transmit the second known signals,data corresponding respectively to antennas prior to increasing thenumber thereof are segmented, and the segmented data are associated toantennas whose number has been increased. The generating may be suchthat while a plurality of subcarriers are being used, the data containedin the burst signal are generated and the data are segmented on asubcarrier-by-subcarrier basis.

The generating may be such that while the number of antennas that shouldtransmit the first known signals is being increased up to the number ofantennas that should transmit the second known signals, datacorresponding respectively to antennas prior to increasing the numberthereof are segmented into the number of increased antennas, and thesegmented data are associated respectively to antennas that shouldtransmit the second known signals. The generating may be such that whilea plurality of subcarriers for at least the second known signal and dataare being used, a combination of subcarriers to be used respectively forthe second known signals are varied for each of the antennas that shouldtransmit the second known signals and when segmented data are associatedrespectively to antennas that should transmit the second known signals,a combination of subcarriers in the second known signals transmittedfrom the same antenna as the data is used for said data.

The method may further comprise determining a data rate of datacontained in the burst signal generated in the generating, wherein thedetermining may set so that a data rate in a case where in thegenerating the number of antennas that should transmit the first knownsignals is increased up to the number of antennas that should transmitthe second known signals is lower than a data rate in a case where inthe generating the number of antennas that should transmit first knownsignals is not increased up to the number of antennas that shouldtransmit the second known signals.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises:transmitting burst signals from a plurality of antennas, respectively;generating burst signals, to be transmitted in the transmitting, whichcontain known signals corresponding respectively to the plurality ofantennas and data assigned posterior to the known signals; anddetermining a data rate of data contained in the burst signals generatedin the generating. When the data correspond to at least one of theplurality of antennas, the generating may be such that said data areassociated to the plurality of antennas by increasing the number ofantennas to be associated thereto, and the determining may be such thatwhen the data are associated to the plurality of antennas, the data rateis determined to be a rate lower than the data rate prior to associatingthe data to the plurality of antennas.

The generating may be such that while using a plurality of subcarriersfor known signals and data, a combination of subcarriers to be usedrespectively for the known signals are varied for each of the pluralityof antennas, and when the data are associated to the plurality ofantennas, a combination of subcarriers in the known signals transmittedfrom the same antenna as the data is used for said data.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises:transmitting burst signals from a plurality of antennas, respectively;and generating burst signals, to be transmitted in the transmitting,which contain known signals corresponding respectively to the pluralityof antennas and data assigned posterior to the known signals. Thegenerating includes: associating data to the plurality of antennas byincreasing the number of antennas to be associated thereto when saiddata correspond to at least one of the plurality of antennas; andvarying a combination of subcarriers to be used respectively for theknown signals, for each of the plurality of antennas while using aplurality of subcarriers, and using a combination of subcarriers in theknown signals transmitted from the same antenna as the data when thedata are associated to the plurality of antennas.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: a generatorwhich generates a burst signal of a plurality of streams that containsfirst known signals and second known signals assigned respectively in aplurality of streams and data assigned in at least one of the pluralityof streams; a deformation unit including: a first means for generatingthe second known signals, where an orthogonal matrix has beenmultiplied, and data which have been increased up to the number of aplurality of streams, by multiplying respectively the second knownsignals and data by the orthogonal matrix, in the burst signal of aplurality of streams generated by the generator; and a second means forvarying the burst signal of a plurality of streams in a manner such thata cyclic time shifting in the orthogonal-matrix-multiplied second knownsignal is executed, with time shift amounts corresponding respectivelyto the plurality of streams, for each stream and at the same time acyclic time shifting in the data which have been increased up to thenumber of a plurality of streams is executed for each stream; and anoutput unit which outputs burst signals of a plurality of streams whichhave been varied by the deformation unit.

The first known signal contained in the burst signal of a plurality ofstreams generated by the generator has a predetermined cycle, and atleast one of the time shift amounts corresponding respectively to theplurality of streams in the deformation unit is greater than or equal tothe predetermined cycle that the first known signal has.

According to this embodiment, even if the number of data streams is lessthan the number of streams in the second known signals, themultiplication by an orthogonal matrix and the cyclic time shiftprocessing are performed, so that the number of data streams can be madeequal to the number of streams in the second known signals. Since thesame processing as with the data streams is also performed on the secondknown signals, the second known signals can be used for a targeted radioapparatus at the time of receiving the data. The same processing as withthe data streams is not performed on the first known signals, so thatthe time shift amount can be made larger and the receivingcharacteristics in the targeted radio apparatus can be improved.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises: generatingsecond known signals, where an orthogonal matrix has been multiplied,and data which have been increased up to the number of a plurality ofstreams, by multiplying respectively the second known signals and databy the orthogonal matrix, in a burst signal of a plurality of streamsthat contains first known signals and the second known signals assignedrespectively in a plurality of streams and the data assigned in at leastone of the plurality of streams; executing, for each stream, a cyclictime shifting in the orthogonal-matrix-multiplied second known signal,with time shift amounts corresponding respectively to the plurality ofstreams, and executing at the same time a cyclic time shifting in thedata which have been increased up to the number of a plurality ofstreams, for each stream; and outputting burst signals of a plurality ofstreams which have been so varied as to contain the second known signaland data to which the cyclic time shifting has been executed. The firstknown signal contained in the burst signal of a plurality of streamsgenerated by the generating has a predetermined cycle, and at least oneof the time shift amounts corresponding respectively to the plurality ofstreams in the executing is greater than or equal to the predeterminedcycle that the first known signal has.

The generating may be such that a plurality of subcarriers are used forburst signals of a plurality of streams and the second known signalsassigned respectively in the plurality of streams use differentsubcarriers for each stream. The outputting may be such that the burstsignals of a plurality of streams which have been varied are outputtedby associating them to the plurality of antennas.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: an outputunit which outputs data assigned in at least one stream, to a targetedradio apparatus compatible with a variable data rate; and a control unitwhich generates a request signal by which to inform said radio apparatusof information on a data rate suitable for a radio channel between saidradio apparatus and the targeted radio apparatus and which outputs thegenerated request signal as the data from the output unit. Whenoutputting the request signal, the output unit also transmits, from aplurality of streams that include streams other than the at least onestream to transmit the data, known signals assigned respectively in theplurality of streams.

According to this embodiment, the known signals are outputted at thetime when the request signal is outputted to the targeted radioapparatus. As are result, information on the data rate, in the targetedradio apparatus, which has been generated based on the known signals canbe obtained, thus improving the accuracy of information.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: a generatorwhich generates a burst signal that contains first known signalsassigned in at least one of a plurality of streams, second known signalsassigned respectively in the plurality of streams and data assigned inthe same stream as the first known signal; and an output unit whichoutputs the burst signal generated by the generator.

According to this embodiment, a stream where the first known signal isto be assigned is identical to that where data is to be assigned. Thus,the estimation result by the first known signal can be used for the dataat a receiving side, thus improving the receiving characteristics ofdata.

The apparatus may further comprise a decision unit which determines adata rate of data contained in the burst signal generated by thegenerator, wherein the decision unit may set so that a data rate in acase where the generator increases the number of streams where the firstknown signals are to be assigned up to the number of a plurality ofstreams is lower than a data rate in a case where the generator does notincrease the number of streams, where the first known signals are to beassigned, up to the number of a plurality of streams.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: an outputunit which outputs a burst signal of a plurality of streams; a generatorwhich generates the burst signal, to be outputted from the output unit,which contains known signals assigned respectively to the plurality ofstreams and data assigned posterior to the known signals; and a decisionunit which determines a data rate of data contained in the burst signalgenerated by the generator. When the data is assigned in at least one ofstreams, the generator assigns said data to the plurality of streams, byincreasing the number of streams to be assigned, and when the generatorassigns said data to the plurality of streams, the determining unitdetermines that the data rate is lower than that prior to assigning thedata to the plurality of streams. According to this embodiment, if thedata are assigned respectively in a plurality of streams and if thecharacteristics of a radio channel from the assigned streams are notsuited for the data transmission, the occurrence of data error can bereduced by lowering the data rate.

While using a plurality of subcarriers for the known signals and data,the generator may be such that a combination of subcarriers to be usedrespectively for the known signals are varied for each of the pluralityof streams, and when the data are assigned in the plurality of streams,a combination of subcarriers in the known signals assigned in the samestream as the data is used for said data.

Still another preferred embodiment according to the present inventionrelates also to a radio apparatus. This apparatus comprises: an outputunit which outputs a burst signal of a plurality of streams; and agenerator which generates the burst signal, to be outputted from theoutput unit, which contains known signals assigned respectively to theplurality of streams and data assigned posterior to the known signals.The generator includes: a first means for assigning data in theplurality of streams, if the data is assigned in at least one of theplurality of streams, by increasing the number of streams to beassigned; and a second means for varying a combination of subcarriers tobe used respectively for the known signals, for each of the plurality ofstreams, while using a plurality of streams for the known signals anddata, and for using a combination of subcarriers in the known signalassigned in the same stream as the data, for said data, when the dataare assigned in the plurality of streams.

According to this embodiment, when the data are assigned in theplurality of streams, the same subcarries are used for the known signaland data assigned in one stream. Thereby the selection of subcarriers tobe used for the respective data can be facilitated.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises: outputtingdata assigned in at least one stream, to a targeted radio apparatuscompatible with a variable data rate; and generating a request signal bywhich to inform a radio apparatus of information on a data rate suitablefor a radio channel between the radio apparatus and the targeted radioapparatus wherein the request signal is outputted as the data from theoutputting. The outputting may be such that when the request signal isoutputted, known signals assigned respectively in a plurality of streamsare also outputted from the plurality of streams that contain streamsother than the at least one stream to transmit the data.

Still another preferred embodiment according to the present inventionrelates also to a transmitting method. This method comprises: generatinga burst signal of a plurality of streams that contains a first knownsignal assigned in at least one of a plurality of streams and secondknown signals assigned respectively in a plurality of streams and dataassigned in the same stream as the first known signal; and outputtingthe generated burst signal. Still another preferred embodiment accordingto the present invention relates also to a transmitting method. Thismethod comprises: outputting a burst signal of a plurality of streams;and generating a burst signal, to be outputted in the outputting, whichcontain known signals assigned respectively in a plurality of streamsand data assigned posterior to the known signal. The generatingincludes: assigning data to a plurality of streams by increasing thenumber of streams to be assigned if the data is assigned in at least oneof the plurality of streams; and varying a combination of subcarriers tobe used respectively for the known signals, for each of a plurality ofstreams, while using the plurality of subcarriers for the known signalsand data, and using a combination of subcarriers in the known signalassigned in the same stream as the data, for said data, when the dataare assigned in the plurality of streams.

The method may further comprise setting by a plurality of antennas atleast one antenna to transmit the data, based on signals received from atargeted radio apparatus, wherein, in the transmitting, the at least oneantenna set by the setting may be used as at least one antenna thatshould transmit the data. The method may further comprise setting atleast one antenna, by which to transmit the data, of antennas selectedin a selecting, based on signals received from a targeted radioapparatus, wherein, in the transmitting, the at least one antenna set bythe setting may be used as at least one antenna that should transmit thedata.

The method may further comprise setting by a plurality of antenna atleast one antenna to transmit the data, based on signals received from atargeted radio apparatus, wherein the generating may be such that the atleast one antenna set in the setting is used as at least one antenna towhich the data is to be associated. The plurality of streams outputtedin the outputting are associated respectively to the plurality ofantennas, and the method may further comprise setting by a plurality ofantennas at least one antenna to output the data, based on signalsreceived from the targeted radio apparatus, wherein the outputting maybe such that a stream associated to the at least one antenna set in thesetting is used as at least one stream that should output the data.

The plurality of streams outputted in the outputting are associatedrespectively to the plurality of antennas, and the method may furthercomprise setting by a plurality of antenna at least one antenna totransmit the data, based on signals received from the targeted radioapparatus, wherein the generating may be such that a stream associatedto the at least one antenna set in the setting is used as at least onestream in which data is to be assigned. The method may further comprisegenerating a burst signal, to be transmitted in the transmitting, thatcontains known signals and data, wherein when data is associated to atleast one of antennas that should transmit the known signals, thegenerating may be such that the amplitude of a signal transmitted fromantennas other than antennas that should transmit the data, among theknown signals, is defined to be a value less than the amplitude of asignal transmitted from the antennas that should transmit the data,among the known signals.

The generating may be such that the amplitude of a signal transmittedfrom antennas other than antennas that should transmit the data, amongthe second known signals, is defined to be a value less than theamplitude of a signal transmitted from the antennas that should transmitthe data, among the second known signals. The generating may be suchthat the amplitude of a signal assigned in a stream other than streamsin which the data are to be assigned, among the second known signals, isdefined to be a value less than the amplitude of a signal assigned inthe streams in which the data are to be assigned, among the second knownsignals. The method may further comprise generating a burst signal, tobe outputted in the outputting, that contains known signals and data,wherein when data is assigned in at lest one of streams in which theknown signals are to be assigned, the generating may be such that theamplitude of a signal assigned in a stream other than streams in whichthe data are to be assigned, among the known signals, is defined to be avalue less than the amplitude of a signal assigned in the streams inwhich the data are to be assigned, among the known signals.

The method may further comprise generating a burst signal, to betransmitted in the transmitting, that contains known signals and data,wherein when data is associated to at least one of antennas that shouldtransmit the known signals, the generating may be such that the numberof subacarriers used at a portion transmitted from antennas other thanantennas that should transmit data, among the known signals, is definedto be a value less than the number of subcarriers used at a portiontransmitted from the antennas that should transmit data, among the knownsignals. The generating may be such that the number of subacarriers usedat a portion transmitted from antennas other than antennas that shouldtransmit data, among the second known signals, is defined to be a valueless than the number of subcarriers used at a portion transmitted fromthe antennas that should transmit data, among the second known signals

The generating may be such that the number of subacarriers used at aportion assigned in a stream other than streams in which the data are tobe assigned, among the second known signals, is defined to be a valueless than the number of subcarriers used at a portion assigned in thestreams in which the data are to be assigned, among the second knownsignals. The method may further comprise generating a burst signal, tobe outputted in the outputting, that contains known signals and data,wherein when data is assigned in at least one of streams in which theknown signals are to be assigned, the generating may be such that thenumber of subacarriers used at a portion assigned in a stream other thanstreams in which the data are to be assigned, among the known signals,is defined to be a value less than the number of subcarriers used at aportion assigned in the streams in which the data are to be assigned,among the known signals. The radio apparatus may further comprise asetting unit which sets at least one antenna to transmit the data, basedon signals received by the plurality of antennas from a targeted radioapparatus, wherein the generator may use the at least one antenna set bythe setting unit as an antenna to which the data is to be associated.The radio apparatus may further comprise a setting unit which sets atleast one antenna, based on signals received by a plurality of antennasfrom a targeted radio apparatus wherein the plurality of streams areassociated respectively to a plurality of streams outputted from theoutput unit, wherein the generator may use streams corresponding to theat least one antenna set by the setting unit, as at least one stream inwhich the data is to be assigned.

In the generator the amplitude of a signal transmitted from antennasother than antennas that should transmit the data, among the secondknown signals, may be defined to be a value less than the amplitude of asignal transmitted from the antennas that should transmit the data,among the second known signals. The amplitude of a signal assigned in astream other than streams in which the data are to be assigned, amongthe second known signals, may be defined to be a value less than theamplitude of a signal assigned in the streams in which the data are tobe assigned, among the second known signals. In the generator the numberof subcarriers used at a portion transmitted from antennas other thanantennas that should transmit the data, among the second known signals,may be defined to be a value less than the number of subcarriers used ata portion transmitted from the antennas that should transmit the data,among the second known signals.

The number of subacarriers used at a portion assigned in a stream otherthan streams in which the data are to be assigned, among the secondknown signals, may be defined to be a value less than the number ofsubcarriers used at a portion assigned in the streams in which the dataare to be assigned, among the second known signals. Still anotherpreferred embodiment according to the present invention relates also toa radio apparatus. This apparatus comprises: a receiver which receives aburst signal that contains: first known signals assigned in at least oneof a plurality of streams; second known signals assigned respectively inthe plurality of streams; and data assigned in the same streams as thefirst known signals; and a processing unit which processes the burstsignal received by the receiver, wherein the receiver receives, atdifferent timings, a portion assigned in a stream in which the firstknown signal is assigned, among the second known signals, and a portionassigned in a stream other than the stream in which the first knownsignal is assigned, among the second known signals. The receiver may seta gain of automatic gain control, based on the first known signal, andreceives respectively, based on the gain, the portion assigned in astream in which the first known signal is assigned, among the secondknown signals, and the portion assigned in a stream other than thestream in which the first known signal is assigned, among the secondknown signals. The processing unit may perform independent operations onthe plurality of streams, respectively.

The radio apparatus may further comprise a setting unit which sets atleast one of a plurality of antennas to transmit the data, by theplurality of antennas, based on signals received from the targeted radioapparatus, wherein the transmitter may use the at least one antenna setby the setting unit, as at least one antenna that should transmit thedata. The radio apparatus may further comprise a setting unit which setsat least one antenna, to transmit the data, which is at least one ofantennas selected by the selector, by at least one of the plurality ofantennas, based on signals received from the targeted radio apparatus,wherein the transmitter may use the at least one antenna set by thesetting unit, as at least one antenna that should transmit the data.

The radio apparatus, wherein the plurality of streams outputted from theoutput unit are associated respectively to a plurality of antennas, mayfurther comprise a setting unit which sets at least one antenna tooutput the data, by the plurality of antennas, based on signals receivedfrom the targeted radio apparatus, wherein the output unit may use astream corresponding to the at least one antenna set by the settingunit, as at least one stream that should output the data. The radioapparatus may further comprise a generator which generates a burstsignal, to be transmitted from the transmitter, that contains knownsignals and data, wherein when the data may be associated to at leastone of antennas that should transmit the known signals, in the generatorthe amplitude of a signal transmitted from antennas other than antennasthat should transmit the data, among the known signals, is defined to bea value less than the amplitude of a signal transmitted from theantennas that should transmit the data, among the known signals.

The radio apparatus may further comprise a generator which generates aburst signal, to be outputted from the output unit, that contains knownsignals and data, wherein when the data is assigned in at least one ofstreams in which the known signals are to be assigned, in the generatorthe amplitude of a signal assigned in a stream other than streams inwhich the data are to be assigned, among the known signals, may bedefined to be a value less than the amplitude of a signal assigned inthe streams in which the data are to be assigned, among the knownsignals. The radio apparatus may further comprise a generator whichgenerates a burst signal, to be transmitted from the transmitter, thatcontains known signals and data, wherein when the data is associated toat least one of antennas that should transmit the known signals, in thegenerator the number of subcarriers used at a portion transmitted fromantennas other than antennas that should transmit the data, among theknown signals, may be defined to be a value less than the number ofsubcarriers used at a portion transmitted from the antennas that shouldtransmit the data, among the known signals.

The radio apparatus may further comprise a generator which generates aburst signal, to be transmitted from the transmitter, that containsknown signals and data, wherein when the data is assigned in at leastone of streams in which the known signals are to be assigned, in thegenerator the number of subcarriers used at a portion assigned in astream other than streams in which the data are to be assigned, amongthe known signals, may be defined to be a value less than the number ofsubcarriers used at a portion assigned in the streams in which the datato be assigned, among the known signals.

It is to be noted that any arbitrary combination of the above-describedstructural components and expressions changed among a method, anapparatus, a system, a recording medium, a computer program and so forthare all effective as and encompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be sub-combinationof these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of examples only, withreference to the accompanying drawings which are meant to be exemplary,not limiting and wherein like elements are numbered alike in severalFigures in which:

FIG. 1 illustrates a spectrum of a multicarrier signal according to afirst embodiment of the present invention.

FIG. 2 illustrates a structure of a communication system according to afirst embodiment of the present invention.

FIGS. 3A and 3B show each a structure of burst format in thecommunication system of FIG. 2.

FIG. 4 shows a sequence of communication procedure to be compared in thecommunication system of FIG. 2.

FIG. 5 shows another sequence of communication procedure to be comparedin the communication system of FIG. 2.

FIG. 6 illustrates a structure of a first radio apparatus of FIG. 2.

FIG. 7 illustrates a structure of frequency-domain signal of FIG. 6.

FIG. 8 illustrates a structure of a first processing unit of FIG. 6.

FIG. 9 is a sequence diagram showing a procedure of setting a data ratein the communication system of FIG. 2.

FIG. 10 is a flowchart showing a procedure of setting a data rate in thefirst radio apparatus of FIG. 6.

FIG. 11 is a sequence diagram showing another procedure of setting adata rate in the communication system of FIG. 2.

FIG. 12 is another flowchart showing a procedure of setting a data ratein a first radio apparatus of FIG. 6.

FIG. 13 is a sequence diagram showing a communication procedure in thecommunication system of FIG. 2.

FIG. 14 is a flowchart showing a transmission procedure in a secondradio apparatus of FIG. 13.

FIG. 15 is a sequence diagram showing still another procedure of settinga data rate in the communication system of FIG. 2.

FIG. 16 is a flowchart showing still another procedure of setting a datarate in the first radio apparatus of FIG. 6.

FIG. 17 illustrates a structure of a control unit shown in FIG. 6.

FIG. 18 illustrates a structure of criteria stored in a storage of FIG.17.

FIGS. 19A and 19B illustrate another structures of burst format in thecommunication system of FIG. 2.

FIG. 20 illustrates still another structure of burst format in thecommunication system of FIG. 2.

FIGS. 21A to 21D illustrate still another structure of burst format inthe communication system of FIG. 2.

FIGS. 22A and 22B illustrate structures of burst format modified overthat of FIG. 20.

FIG. 23 is a flowchart showing a transmission procedure corresponding tothe burst formats shown in FIGS. 22A and 22B.

FIG. 24 is a flowchart showing another transmission procedurecorresponding to the burst formats shown in FIGS. 22A and 22B.

FIG. 25 illustrates a structure of a transmitting apparatus according toa second embodiment of the present invention.

FIGS. 26A and 26B each illustrate a burst format of a burst signalgenerated in the transmitting apparatus of FIG. 25.

FIG. 27 illustrates a structure of a burst format according to a thirdembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on the following embodimentswhich do not intend to limit the scope of the present invention butexemplify the invention. All of the features and the combinationsthereof described in the embodiments are not necessarily essential tothe invention.

First Embodiment

Before describing the present invention in detail, an outline of thepresent invention will be described first. A first embodiment of thepresent invention relates to a MIMO system comprised of two radioapparatuses (for convenience, hereinafter referred to as “first radioapparatus” and “second radio apparatus”). Both the first radio apparatusand the second radio apparatus carry out adaptive array signalprocessing. The MIMO system changes the data rate by varying the valuesor mode corresponding to the number of antennas, modulation scheme andcoding rate of error correction. In so doing, a radio apparatus at thetransmitting side transmits a rate request signal to a radio apparatusat the receiving side. For example, when the first radio apparatustransmits data to the second radio apparatus, the first radio apparatustransmits the rate request signal to the second radio apparatus.

The second radio apparatus conveys rate information of its own to thefirst radio apparatus. However, the rate information contains errors inthe following cases. The first example is a case where some period oftime is required and spent after the second radio apparatus hasdetermined the rate information. The second example is a case where thenumber of antennas used for transmission in the first radio apparatusdiffers between when the second radio apparatus has determined the rateinformation and when it receives the data from the first radioapparatus. Specific explanation for these cases will be given later. Inorder for the first radio apparatus of the present embodiment to obtaininformation which is as accurate as possible, from the second radioapparatus, a training signal is also added when the rate request signalis transmitted from the first radio apparatus. As a result thereof, thesecond radio apparatus can update the rate information by the trainingsignal, so that the rate information will be accurate.

When the data is to be transmitted from the first radio apparatus to thesecond radio apparatus, the first radio apparatus must derive, inadvance, transmission weight vectors based on a training signal. It isfor this reason that the first radio apparatus requests the second radioapparatus to send the training signal (hereinafter, the signal for thisrequest will be referred to as “training request signal”). In accordancewith this training request signal, the second radio apparatus transmitsthe training signal to the first radio apparatus. At this time, insteadof transmitting the training signal from all of the antennas of thesecond radio apparatus, the second radio apparatus sends the trainingsignal from an antenna that should receive data from the first radioapparatus, to reduce the power consumption.

FIG. 1 illustrates a spectrum of a multicarrier signal according to afirst embodiment of the present invention. In particular, FIG. 1 shows aspectrum of a signal in the OFDM modulation scheme. One of a pluralityof carriers in an OFDM modulation scheme is generally called asubcarrier. Herein, however, each of the subcarriers is designated by a“subcarrier number”. Similar to the IEEE802.11a standard, 53subcarriers, namely, subcarrier numbers “−26” to “26” are definedherein. It is to be noted that the subcarrier number “0” is set to nullso as to reduce the effect of a direct current component in a basebandsignal. The respective subcarriers are modulated by a modulation schemewhich is set variably. Used here is any of modulation schemes among BPSK(Binary Phase-Shift Keying), QPSK (Quadrature Phase-Shift Keying), 16QAM(Quadrature Amplitude Modulation) and 64QAM. Convolutional coding isapplied, as an error correction scheme, to these signals. The codingrates for the convolutional coding are set to ½, ¾ and so forth. Thenumber of antennas used in a MIMO system is set variably. As a result,when the mode or values corresponding to the modulation scheme, codingrate and the number of antennas are set variably, the data rate is alsoset variably. Hereinafter, the information on data rates will bereferred to as “rate information” as mentioned already, and each rateinformation includes values corresponding respectively to the modulationscheme, coding rate and the number of antennas. Unless otherwiseparticularly necessary, the description on values of the modulationscheme, coding rate and the number of antennas will not be given herein.

FIG. 2 illustrates a structure of a communication system according tothe first embodiment of the present invention. A communication system100 includes a first radio apparatus 10 a and a second radio apparatus10 b, which is generically called “radio apparatus 10”. The first radioapparatus 10 a includes a first antenna 12 a, a second antenna 12 b, athird antenna 12 c and a fourth antenna 12 d, which are referred to as“antennas 12”, and the second radio apparatus 10 b includes a firstantenna 14 a, a second antenna 14 b, a third antenna 14 c and a fourthantenna 14 d, which are generically referred to as “antennas 14”. One ofthe first radio apparatus 10 a and the second radio apparatus 10 bcorresponds to a transmitting apparatus, whereas the other correspondsto a receiving apparatus. One of the first radio apparatus 10 a and thesecond radio apparatus 10 b corresponds to a base station apparatus,whereas the other corresponds to a terminal apparatus.

Before describing a structure of the communication system 100, anoutline of a MIMO system will be explained first. Assume herein thatdata are being transmitted from the first radio apparatus 10 a to thesecond radio apparatus 10 b. The first radio apparatus 10 a transmitsdifferent data from the first antenna 12 a to fourth antenna 12 d,respectively. As a result, the data rate becomes higher. The secondradio apparatus 10 b receives the data by the first antenna 14 to fourthantenna 14 d. The second radio apparatus 10 b separates the receivedsignals by adaptive array signal processing and demodulates the signalstransmitted from the first antenna 12 a to fourth antenna 12 bindependently.

Since the number of antennas 12 is “4” and the number of antennas 14 isalso “4”, the number of combinations of channels between the antennas 12and the antennas 14 is “16”. The channel characteristic between from theith antenna 12 i to the jth antenna 14 j is denoted by h_(ij). In FIG.2, the channel characteristic between the first antenna 12 a and thefirst antenna 14 a is denoted by h₁₁, that between the first antenna 12a and the second receiving antenna 14 b by h₁₂, that between the secondantenna 12 b and the first antenna 14 a by h₂₁, that between the secondantenna 12 b and the second antenna 14 b by h₂₂, and that between thefourth antenna 12 d and the fourth antenna 14 d by h₄₄. For the clarityof illustration, it is omitted to show the other channels in FIG. 2.

The second radio apparatus 10 b operates so that data transmitted fromthe first antenna 12 a and the second antenna 12 b, respectively, aredemodulated independently by adaptive array signal processing. The firstradio apparatus 10 a also performs adaptive array signal processing onthe first antenna 12 a to the fourth antenna 12 d. In this manner,adaptive array signal processing is performed also at the transmittingside, namely, by the first radio apparatus 10 a, so that the spacedivision in a MIMO system is ensured. As a result, the interference ofsignals transmitted by a plurality of antennas 12 becomes smaller, sothat the data transmission characteristics can be improved.

The first radio apparatus 10 a transmits different data respectivelyfrom the first antenna 12 a to fourth antenna 12 d. The first radioapparatus 10 a controls the number of antenna 12 a to be used, inresponse to the rate or capacity of data to be transmitted. Forinstance, if the volume of data is larger, “four” antennas 12 are used,whereas if the volume of data is small, “two” antennas 12 are used. Whenthe first radio apparatus 10 a decides on the number of antennas 12 tobe used, the rate information in the second radio apparatus lob isreferred to. For instance, when the receiving by “two” antennas 14 isinstructed from the second radio apparatus 10 b, the first radioapparatus 10 a uses “two” antennas 12. When the first radio apparatus 10a transmits data, it performs adaptive array signal processing on theantennas 12. As a result, the first radio apparatus 10 a receivesbeforehand a training signal from the second radio apparatus 10 b andderives transmission weight vectors based on the training signal.

The second radio apparatus 10 b performs adaptive array signalprocessing on the first antenna 14 a to fourth antenna 14 d and thenreceives data from the first radio apparatus 10 a. As described above,the second radio apparatus 10 b conveys the rate information andtransmits the training signal to the first radio apparatus 10 a. It isto be noted that the operations by the first radio apparatus 10 a andthe second apparatus 10 b may be reversed.

FIGS. 3A and 3B show each a structure of burst format in a communicationsystem 100. FIG. 3A shows a burst format when the number of antennas 12used is “2”. The upper row of FIG. 3A shows a burst signal transmittedfrom the first antenna 12 a whereas the lower row thereof shows a burstsignal transmitted from the second antenna 12 b. “Legacy STS (ShortTraining Sequence)”, “Legacy LTS (Long Training Sequence)” and “LegacySignal” are signals compatible with a communication system, such as awireless LAN system that conforms to the IEEE802.11a standard, which isnot compatible with a MIMO. “Legacy STS” is used for timingsynchronization, AGC (Automatic Gain Control) and the like, “Legacy LTS”is used for channel estimation and “Legacy Signal” contains controlinformation. Signals assigned posterior to “MIMO Signal” are thosecharacteristic of and inherent to a MIMO system, and “MIMO Signal”contains control information corresponding to a MIMO system. “FirstMIMO-STS” and the “Second MIMO-LTS” are used for timing synchronization,AGC and the like, “First MIMO-LTS” and “Second MIMO-LTS” are used forchannel estimation, and “First Data” and the “Second Data” are data tobe transmitted.

Similar to FIG. 3A, FIG. 3B shows a burst format at the time when “two”antennas 12 are used for data transmission. In FIG. 3B, however, theabove-described training signals are added. In FIG. 3B, the trainingsignals correspond to “First MIMO-STS”, “First MIMO-LTS” through “FourthMIMO-STS” and “Fourth MIMO-LTS”. The “First MIMO-STS”, “First MIMO-LTS”through “Fourth MIMO-STS” and “Fourth MIMO-LTS” are transmitted from thefirst antenna 12 to fourth antenna 12 d, respectively. As mentionedearlier, the number of antennas 12 from which the training signals aretransmitted may be less than “4”. “First MIMO-STS” to “Fourth MIMO-STS”are structured by patterns such that the interference among them becomessmall. The same is true for “First MIMO-LTS” to “Fourth MIMO-LTS”. Theexplanation of these structures thereof is omitted here. Though it maybe generally a case that “Legacy LTS”, “First MIMO-LTS” and the like inFIG. 3A are called training signals, the training signals defined inthis patent specification are restricted to the aforementioned trainingsignals as shown in FIG. 3B. That is, “training signals” correspond to“MIMO-LTSs” having multiple streams the number of which corresponds tochannels to be estimated, in order for a targeted radio apparatus 10 toestimate the channels, independently of the number of data to betransmitted, namely, the number of streams. Hereinafter, the “FirstMIMO-STS” to “Fourth MIMO-LTS” are generically referred to as “MIMO-STS”or “MIMO-STSs”, whereas “First Data” and “Second Data” are genericallyreferred to as “data” or “Data”.

FIG. 4 shows a sequence of communication procedure to be compared in acommunication system 100. Shown here is an operation in which the firstradio apparatus 10 a acquires information on rates of the second radioapparatus 10 b. For the brevity of explanation, the operation foradaptive array signal processing is omitted here. The first radioapparatus 10 a sends a rate request signal to the second radio apparatus10 b (S10). The second radio apparatus 10 b sends rate information tothe first radio apparatus 10 a (S12). The first radio apparatus 10 asets a data rate, based on the rate information (S14). That is, the datarate is set by referring to the rate information. The first radioapparatus 10 a transmits data at the data rate thus set (S16). Thesecond radio apparatus 10 b performs a receiving processing on the data(S18).

According to the above-described operation, the rate information in thesecond radio apparatus 10 b contains errors, as described above, in thefollowing cases. First one is a case where a certain period of timeelapses after the second radio apparatus 10 b has determined the rateinformation. In other words, the characteristics of a channel betweenthe first radio apparatus 10 a and the second radio apparatus 10 bgenerally fluctuates, and the content of rate information also variesaccording to the fluctuation of channel characteristics. For example,there is a case where although the receiving at 50 Mbps was possiblewhen the rate information was decided, the receiving at 10 MBps is thelimit when data are received from the first radio apparatus 10 a. Thesecond one is a case where the number of antennas used differs betweenwhen the second radio apparatus 10 b decides on the rate information andwhen the data are received from the first radio apparatus 10 a. In otherwords, when the training signals have not yet been fully received fromall of the antennas 12 but the second apparatus 10 b determines the rateinformation, there exists an unrecognized channel, so that accurate rateinformation cannot be derived. For example, when rate information isderived from the first antenna 12 a and the second antenna 12 b, theeffect of the third antenna 12 c and fourth antenna 12 d is not takeninto account, so that error will be contained in the rate information.

FIG. 5 shows another sequence of communication procedure to be comparedin the communication system 100. Shown here is an operation in whichdata are transmitted by MIMO. The first radio apparatus 10 a sends atraining request signal to the second radio apparatus 10 b (S20). Thetraining request signal is contained in the “First Data” and/or “SecondData” shown in FIG. 3A. The second radio apparatus 10 b sends a trainingsignal to the first radio apparatus 10 a (S22). The first radioapparatus 10 a derives transmission weight vectors, based on thetraining signals received and then sets them (S24). The first radioapparatus 10 a transmits data using the transmission weight vectors(S26). The second radio apparatus derives receiving weight vectors forthe received data and sets them (S28) Then the second radio apparatus 10b performs a receiving processing on the data, based on the receivingweight vectors (S30).

According to the above-described operation, the second radio apparatus10 b transmits the training signals from all of antennas 14, so that thepower consumption increases. On the other hand, there is a case wherethe less number of antennas 14 to be used suffices if the data rate inthe rate information is low to some extent. In such a case, thedeterioration of transmission quality can be suppressed even if notraining signal is sent from the antennas 14 which are not scheduled tobe used. In particular, the reduction of power consumption is desiredwhen the second radio apparatus 10 b is a terminal apparatus and isbattery-driven.

FIG. 6 illustrates a structure of a first radio apparatus 10 a. Thefirst radio apparatus 10 a includes a first radio unit 20 a, a secondradio unit 20 b, . . . and a fourth radio unit 20 d, which aregenerically referred to as “radio unit 20”, a first processing unit 22a, a second processing unit 22 b, . . . and a fourth radio 22 d, whichare generically referred to as “processing unit 22”, a first modem unit24 a, a second modem unit 24 b, . . . and a fourth modem unit 24 d,which are generically referred to as “modem unit 24”, an IF unit 26, aselector 28, a control unit 30 and a rate information managing unit 32.Signals involved include a first time-domain signal 200 a, a secondtime-domain signal 200 b, . . . and a fourth time-domain signal 200 d,which are generically referred to as “time-domain signal 200”, and afirst frequency-domain signal 202 a, a second frequency-domain signal202 b, . . . and a fourth frequency-domain signal 202 d, which aregenerically referred to as “frequency-domain signal 202”. The secondradio apparatus 10 b has a structure similar to that of the first radioapparatus 10 a. Different component or components will be furtherincluded in this structure depending on whether the first radioapparatus 10 a (or second radio apparatus 10 b) is a base stationapparatus or terminal apparatus. However, for the clarity ofexplanation, they will be omitted here.

As a receiving operation, the radio unit 20 carries out frequencyconversion of received radiofrequency signal received by the antennas 12so as to derive baseband signals. The radio unit 20 outputs the basebandsignals to the processing unit 22 as the time-domain signals 200. Thebaseband signal, which is composed of in-phase components and quadraturecomponents, shall generally be transmitted by two signal lines. For theclarity of figure, the signal is presented here by a single signal line.An AGC unit and/or an A-D conversion unit are also included. As atransmission operation, the radio unit 20 carries out frequencyconversion of baseband signals from the processing unit 22 so as toderive radiofrequency signals. Here, the baseband signal from theprocessing unit 22 is also indicated as the time-domain signal 200. Theradio unit 20 outputs the radiofrequency signals to the antenna 12. Apower amplifier and/or a D-A conversion unit are also included. It isassumed herein that the time-domain signal 200 is a multicarrier signalconverted to the time domain and is a digital signal. Signals processedin the radio unit 20 form burst signals, and their formats are those asshown in FIGS. 3A and 3B.

As a receiving operation, the processing unit 22 converts a plurality oftime-domain signals 200 respectively into the frequency domain andperforms adaptive array signal processing on the thus convertedfrequency-domain signals. Then the processing unit 22 outputs the resultof adaptive array signal processing as the frequency-domain signals 202.One frequency-domain signal 202 corresponds to a signal transmitted fromone of the antennas 14 shown in FIG. 2, and this corresponds to a signalcorresponding to one channel. As a transmission operation, theprocessing unit 22 inputs, from the modem unit 24, the frequency-domainsignal 202 serving as a frequency-domain signal, and then performsadaptive array signal processing on the frequency-domain signal. Thenthe processing unit 22 coverts the signal that has undergone theadaptive array signal processing, into the time domain and outputs thethus converted signal as a time-domain signal 200. Here, the number ofantennas 12 to be used in the transmission processing is specified bythe control unit 30. It is assumed herein that the frequency-domainsignal 202, which is a signal in the frequency domain, contains aplurality of subcarrier components. For the clarity of figure, thefrequency-domain signal is arranged in the order of the subcarriernumbers, and forms serial signals.

FIG. 7 illustrates a structure of a frequency-domain signal. Assumeherein that a combination of subcarrier numbers “−26” to “26” shown inFIG. 1 constitutes an “OFDM” symbol. An “i”th OFDM symbol is such thatsubcarrier numbers “1” to “26” and subcarriers “−26” to “−1” arearranged in this order. Assume also that an “i−1”th OFDM symbol isplaced immediately before the “i”th OFDM symbol, and an “i+1”th OFDMsymbol is placed immediately after the “i”th OFDM symbol.

Referring back to FIG. 6, as a receiving processing, the modem unit 24demodulates and decodes the frequency-domain signal 202 outputted fromthe processing unit 22. The demodulation and decoding are carried outper subcarrier. The modem unit 24 outputs the demodulated signal to theIF unit 26. As a transmission processing, the modem unit 24 carries outcoding and modulation. The modem unit 24 outputs the modulated signal tothe processing unit 22 as a frequency-domain signal 202. When thetransmission processing is carried out, the modulation scheme and codingrate are specified by the control unit 30. They are specified based onthe above-described rate information.

As a receiving processing, the IF unit 26 combines signals outputtedfrom a plurality of modem units 24 and then forms one data stream. TheIF unit 26 outputs the data stream. As a transmission processing, the IFunit 26 inputs one data stream and then separates it. Then the IF unit26 outputs the thus separated data to a plurality of modem units 24.

A description is given hereinbelow of a case when a request signal istransmitted in such a structure as above. As shown in FIG. 3A or 3B, theprocessing unit 22 transmits, from at least one of a plurality ofantennas 12, data corresponding respectively to the plurality ofantennas 12. If the number of antennas 12 to be used is “2”, the datacorrespond to “First Data” and “Second Data” in FIG. 3A or 3B. Assumeherein that the number of antennas 12 to be used for data transmissionis specified by the control unit 30. The processing unit 22 adds signalsother than “Legacy STS” and the like as shown in FIG. 3A. When thenumber of antennas 12 to be used for data transmission is “4”, “ThirdData” and “Fourth Data” which are not shown in FIGS. 3A and 3B will beadded. Such data are transmitted to the second radio apparatus 10 bcompatible with variable data rates.

The control unit 30 generates request signals with which to let thesecond radio apparatus 10 b supply information on rates at the secondradio apparatus 10 b. Then the control signal 30 outputs the thusgenerated request signal to the modulation unit 24. When transmittingthe request signal, the processing unit 22 also transmits, from aplurality of antennas 12 which includes antennas 12 other than theantennas 12 to transmit the data, known signals correspondingrespectively to the plurality of antennas 12. Here, the request signalis allotted to “first data” and/or “second data” of FIG. 3B. In FIG. 3B,the known signals correspond to “First MIMO-STS”, “First MIMO-LTS” to“Fourth MIMO-STS” and “Fourth MIMO-LTS”. As a result, even if the numberof antennas 12 to transmit data is “2” as in the case of FIG. 3B, theprocessing unit 22 transmits the known signals, namely, trainingsignals, from “four” antennas 12. In this manner, the request signalsand the training signals are combined together and transmitted, and thenthe first radio apparatus 10 a has the second radio apparatus 10 bgenerate the rate information based on the training signals, and canobtain the thus generated rate information. As a result, the accuracy ofrate information, acquired by the first radio apparatus 10 a, on thesecond radio apparatus 10 b improves.

In response to the above description, a case where the request signaland the training signals are received will be described hereinbelow. Thecontrol unit 30 generates the rate information, based on the receivedtraining signal. A method for generating the rate information may bearbitrary. For example, the rate information may be generated in amanner such that the signal strength of signals received by the radiounit 20 is measured and the measured signal strength is compared with athreshold value. Alternatively, the rate information may also begenerated based on the receiving weight vectors derived by theprocessing unit 22. More detailed description of a specific example togenerate the rate information will be given later. The rate informationmay be generated based on a demodulation result obtained by the modemunit 24. The rate information thus determined is transmitted via themodem unit 24, processing unit 22 and radio unit 20 and is at the sametime stored in the rate information managing unit 32. The rateinformation managing unit 32 also stores the rate information at atargeted radio apparatus 10.

With a structure described as above, the first radio apparatus 10 aoperates as follows to reduce the power consumption. The radio unit 20receives, via a plurality of antennas 12, training signals from thesecond radio apparatus 10 b. Based on the received training signals, theselector 28 selects, from among a plurality of antennas 12, at least oneantenna to be used when data are received from the second radioapparatus 10 b. More specifically, such an operation is as follows.Based on the training signals received by the radio unit 20, theselector 28 derives signal strength corresponding respectively to theplurality of antennas 12. The selector 28 preferentially selectsantennas 12 whose strength is larger. If, for example, the number ofantennas 12 to be used when the data are received is “3”, the selector28 selects “three” antennas from among those whose signal strengths arelarge. It is to be noted here that the total number of antennas 12 to beselected is specified separately based on a value of data rate, at whichthe data are to be transmitted, and a value of power consumption. Whileusing the antennas 12 selected by the selector 28, the processing unit22 transmits the training signals. In this manner, the power consumptionis lowered by reducing the number of the antennas 12 that shouldactually transmit the training signal.

It is also possible to execute the above-described operation even in acase when the request signal is not transmitted. In other words, theabove-described operation can be applied even in a case when a trainingrequest signal is accepted from the second radio apparatus 10 b. Thatis, the selector 28 selects, from among a plurality of antennas 12, atleast one antenna to be used when the data from the second radioapparatus 10 b are received. In so doing, the selection is done based onan instruction from the control unit 30. The processing unit 22transmits data corresponding respectively to the antennas 12, from atleast one of a plurality of antennas 12 to the second radio apparatus 10b, and also transmits training signals corresponding respectively to theantennas 12 selected by the selector 28, independently of the number ofantennas 12 to be used when the data are transmitted. For example, thedata are transmitted from “two” antennas 12 and the training signals aretransmitted from “three” antennas 12.

In terms of hardware, this structure can be realized by a CPU, a memoryand other LSIs of an arbitrary computer. In terms of software, it isrealized by memory-loaded programs which have managing and schedulingfunctions or the like, but drawn and described herein are functionblocks that are realized in cooperation with those. Thus, it isunderstood by those skilled in the art that these function blocks can berealized in a variety of forms such as by hardware only, software onlyor the combination thereof.

FIG. 8 illustrates a structure of a first processing unit 22 a. Thefirst processing unit 22 a includes an FFT (Fast Fourier Transform) unit40, a synthesis unit 42, a reference signal generator 44, a receivingweight vector computing unit 54, a separation unit 46, a transmissionweight vector computing unit 52, an IFFT unit 48 and a preamble addingunit 50. The synthesis unit 42 includes a first multiplier 56 a, asecond multiplier 56 b, . . . and a fourth multiplier 56 d, which aregenerically referred to as “multiplier 56”, and an adder 60. Theseparation unit 46 includes a first multiplier 58 a, a second multiplier58 b, . . . and a fourth multiplier 58 d, which are generically referredto as “multiplier 58”.

The FFT unit 40 inputs a plurality of time-domain signals 200 andperforms Fast Fourier Transform on them, respectively, so as to derivefrequency-domain signals. As described earlier, one frequency-domainsignal is such that signals corresponding to subcarriers are arrangedserially in the order of the subcarrier numbers.

The multiplier 56 weights the frequency-domain signal with a receivingweight vector outputted from the receiving weight vector computing unit54, and the adder 60 adds up the outputs from the multipliers 56. Sincethe frequency-domain signals are arranged in the order of the subcarriernumbers, the receiving weight vectors outputted from the receivingweight vector computing unit 54 are arranged in such a manner as tocorrespond thereto, too. That is, one multiplier 56 inputs successivelythe receiving weight vectors arranged in the order of the subcarriernumbers. Thus, the adder adds up a multiplication result on asubcarrier-by-subcarrier basis. As a result, the added-up signal is alsoarranged serially in the order of the subcarrier numbers as shown inFIG. 7. The thus added signal is the aforementioned frequency-domainsignal 202.

In the following explanation, if the signal to be processed correspondsto the frequency-domain, the processing therefor is basically executedsubcarrier by subcarrier, too. For the brevity of explanation, theprocessing for one subcarrier will be described herein. Hence, theprocessing for a plurality of subcarriers will be accommodated in amanner such that the processing for a single subcarrier is executed inparallel or serially.

During the period of “Legacy STS”, “Legacy LTS”, “First MIMO-STS” and“First MIMO-LTS”, the reference signal generator 44 outputs, asreference signals, “Legacy STS”, “Legacy LTS”, “First MIMO-STS” and“First MIMO-LTS” which have been stored beforehand. During the periodother than these periods, the frequency-domain signal 202 is determinedby a predefined threshold value, and its result is outputted as areference signal. The determination may be a soft decision instead ofthe hard decision.

The receiving weight vector computing unit 54 derives receiving weightvectors, based on the frequency-domain signal outputted from the FFTunit 40 and the reference signal. A method for deriving the receivingweight vectors may be arbitrary. One such a method is the derivation byan LMS (Least Mean Square) algorithm. The receiving weight vectors maybe derived by a correlation processing. When a correlation processing iscarried out, the frequency-domain signal and the reference signal willbe inputted not only from the first processing unit 22 a but also fromthe second processing unit 22 b via a signal line not shown. If afrequency-domain signal in the first processing unit 22 a is denoted byx₁(t), a frequency-domain signal in the second processing unit 22 b byx₂(t), a reference signal in the first processing unit 22 a by S₁(t) anda reference signal in the second processing 22 b by S₂(t), then x₁(t)and x₂(t) will be expressed by the following Equation (1):

x ₁(t)=h ₁₁ S ₁(t)+h ₂₁ S ₂(t)

x ₂(t)=h ₁₂ S ₁(t)+h ₂₂ S ₂(t)  (1)

The noise is ignored here. A first correlation matrix R₁, with E as anensemble average, is expressed by the following Equation (2):

$\begin{matrix}{R_{1} = \begin{bmatrix}{E\left\lbrack {x_{1}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {x_{1}S_{1}^{*}} \right\rbrack} \\{E\left\lbrack {x_{2}S_{2}^{*}} \right\rbrack} & {E\left\lbrack {x_{2}S_{2}^{*}} \right\rbrack}\end{bmatrix}} & (2)\end{matrix}$

A second correlation matrix R₂ among the reference signals is given bythe following Equation (3):

$\begin{matrix}{R_{2} = \begin{bmatrix}{E\left\lbrack {S_{1}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {S_{1}S_{2}^{*}} \right\rbrack} \\{E\left\lbrack {S_{2}S_{1}^{*}} \right\rbrack} & {E\left\lbrack {S_{2}S_{2}^{*}} \right\rbrack}\end{bmatrix}} & (3)\end{matrix}$

Finally, the first correlation matrix R₁ is multiplied by the inversematrix of the second correlation matrix R₂ so as to derive a receivingresponse vector, which is expressed by the following Equation (4):

$\begin{matrix}{\begin{bmatrix}h_{11} & h_{21} \\h_{12} & h_{22}\end{bmatrix} = {R_{1}R_{2}^{- 1}}} & (4)\end{matrix}$

Then the receiving weight vector computing unit 54 computes a receivingweight vector from the receiving response vector.

The transmission weight vector computing unit 52 estimates thetransmission weight vectors necessary for weighting the frequency-domainsignals 202, from the receiving weight vectors. The method forestimating the transmission weight vectors is arbitrary. As a mostsimple method therefor, however, the receiving weight vector may be usedintact. As another method, the receiving weight vector may be correctedusing a conventional technique in view of the Doppler frequency shift ofa propagation environment caused by time difference in between areceiving processing and a transmission processing. Here, it is assumedthat the receiving weight vectors are used, directly and withoutmodification, as the transmission weight vectors.

The multipliers 58 weight the frequency-domain signals 202 with thetransmission weight vectors, and the results thereof are outputted tothe IFFT unit 48. Then the IFFT unit 48 performs inverse Fast FourierTransform on the signals outputted from the multipliers 58 so as toconvert them into time-domain signals. As shown in FIGS. 3A and 3B, thepreamble adding unit 50 adds preambles in a header portion of burstsignal. Here, “Legacy STS”, “Legacy LTS”, “First MIMO-STS” and “FirstMIMO-LTS” are added. The preamble adding unit 50 outputs, as time-domainsignals 200, the signals where the preamble has been added. Theabove-described operation is controlled by the control unit 30 shown inFIG. 6. In FIG. 8, the first time-domain signal 200 a and the likeappear twice. However, these are the signal in one direction and thesecorrespond to the first time-domain signal 200 a and the like which aretwo-way signals as shown in FIG. 6.

An operation of a communication system 100 structured as above will bedescribed. FIG. 9 is a sequence diagram showing a procedure of setting adata rate in the communication system 100. FIG. 9 is a sequence diagramthat shows a case when a rate request signal and training signals aretransmitted, and FIG. 9 corresponds to FIG. 4. The first radio apparatus10 a transmits to the second radio apparatus 10 b a rate request signaland training signals as shown in FIG. 3B (S40). The second radioapparatus 10 b estimates a channel based on the training signals (S42).Here, the channel estimation corresponds to deriving the aforementionedreceiving weight vectors. The second radio apparatus 10 b updates therate information, based on the estimated channel (S44). The descriptionon updating the rate is omitted here. The second radio apparatus 10 btransmits the rate information to the first radio apparatus 10 a (S46).The first radio apparatus 10 a sets a data rate by referring to the thusreceived rate information (S48).

FIG. 10 is a flowchart showing a procedure of setting a data rate in thefirst radio apparatus 10 a. FIG. 10 corresponds to the operation offirst radio apparatus 10 a in FIG. 9. The processing unit 22 transmits arate request signal in a format of training signals as shown in FIG. 3B(S50). If the IF unit 26 does not accept rate information via theantenna 12, radio unit 20, processing unit 22 and modem unit 24 (N ofS52), keep waiting until the IF unit 26 accepts it. If, on the otherhand, the IF unit 26 accepts the rate information (Y of S52), thecontrol unit 30 sets a data rate (S54). The rate information managingunit 32 stores the rate information.

FIG. 11 is a sequence diagram showing another procedure of setting adata rate in the communication system 100. FIG. 11, which corresponds toFIG. 5, is a sequence diagram for a processing wherein adaptive arraysignal processing is taken into account and a lower power consumption isintended on top of FIG. 9. The first radio apparatus 10 a transmits atraining request signal to the second radio apparatus 10 b (S60). Thesecond radio apparatus 10 b transmits training signals to the firstradio apparatus 10 a (S62). The first radio apparatus 10 a selectsantenna 12 based on the strength of the received training signals (S64).The first radio apparatus 10 a transmits to the second radio apparatus10 b a rate request signal and training signals as shown in FIG. 3B(S66). The training signals are transmitted from the selected antenna12.

The second radio apparatus 10 b estimates a channel, based on thetraining signals (S68). Based on the estimated channel, the second radioapparatus 10 b updates the rate information (S70). The second radioapparatus 10 b derives transmission weight vectors and then sets them(S72). The second radio apparatus 10 b transmits the rate information tothe first radio apparatus 10 a (S74). In so doing, the transmissionweight vectors are used so as to execute adaptive array signalprocessing. The first radio apparatus unit 10 a sets receiving weightvectors, based on a burst signal that contains the rate information(S76). While using the receiving weight vectors, the first radioapparatus 10 a then performs a receiving processing on the rateinformation (S78). The first radio apparatus 10 a sets a data rate byreferring to the accepted rate information (S80).

FIG. 12 is a flowchart showing another procedure of setting a data ratein the first radio apparatus 10 a. FIG. 12 corresponds to the operationof first radio apparatus 10 a shown in FIG. 11. The processing unit 22transmits a training request signal (S90). The radio unit 20 receivestraining signals (S92). The selector 28 measures the strength of thereceived training signals for each antenna 12, and selects an antenna 12based on the measured strength (S94). The processing unit 22 transmits,from the selected antenna 12, the training signals in a format oftraining signals as shown in FIG. 3B and also transmits a rate requestsignal (S96).

If the IF unit 26 does not accept the rate information via the antenna12, radio unit 20, processing unit 22 and modem unit 24 (N of S98), keepwaiting until the IF unit 26 accepts it. If, on the other hand, the IFunit 26 accepts the rate information (Y of S98), the processing unit 22sets receiving weight vectors (S100). The processing unit 22, modem unit24 and IF unit 26 carry out receiving processing (S102). The controlunit 30 sets a data rate (S104). The rate information managing unit 32stores the rate information.

FIG. 13 is a sequence diagram showing a communication procedure in thecommunication system 100. FIG. 13 is a sequence diagram for a procedurewherein the lower power consumption is intended in transmitting thetraining signals. The first radio apparatus 10 a transmits a trainingrequest signal to the second radio apparatus 10 b (S110). The secondradio apparatus 10 b selects an antenna 14 which is to be used when dataare received (S112). The second radio apparatus 10 b transmits trainingsignals to the first radio apparatus from the selected antenna 14(S114). The first radio apparatus 10 a sets transmission weight vectors,based on the strength of the received training signals (S116). Whileusing the transmission weight vectors, the first radio apparatus 10 atransmits data to the second radio apparatus 10 b (S118). The secondradio apparatus 10 b derives receiving weight vectors from a burstsignal that contains the data, and then sets this (S120). A receivingprocessing is carried out based on the receiving weight vectors (S122)

FIG. 14 is a flowchart showing a transmission procedure in the secondradio apparatus 10 b. FIG. 14 corresponds to the operation of the secondradio apparatus 10 b of FIG. 13. The processing is not started if the IFunit 26 does not accept a training request signal via the antenna 12,radio unit 20, processing unit 22 and modem unit 24 (N of S130). If, onthe other hand, the IF unit 26 accepts the training request signal (Y ofS130), the control unit 30 selects an antenna 14 which is to be used atthe time of receiving (S132). The processing unit 22 transmits trainingsignals from the selected antennas 14 (S134).

In the present embodiment described so far, the first radio apparatus 10a does not carry out adaptive array signal processing, namely, beamforming at the time of sending the training signals. This is for thepurpose of having the second radio apparatus 10 b perform the channelestimation in a state where the directivity of antenna isomnidirectional. In other words, this is for the purpose of having thesecond radio apparatus 10 b perform the channel estimation in a stateclose to that of the channel in which the antennas are omnidirectional.As described earlier, if the training signals and the rate requestsignal are combined together, the first radio apparatus 10 a canprocess, at high speed, the rate information determined in the secondradio apparatus 10 b by performing the following processing. If thefirst radio apparatus 10 a performs beamforming, SNR (Signal-to-NoiseRatio) in the second radio apparatus 10 b at the time of receiving canbe improved compared to the case when it does not perform beamforming.If the second radio apparatus 10 b determines the data rate based on theSNR, the improved SNR makes the determined data rate higher. Thus, whensending the rate request signal, the first radio apparatus 10 a hereperforms beamforming at least on the training signals.

FIG. 15 is a sequence diagram showing still another procedure of settinga data rate in the communication system 100. The second radio apparatus10 b transmits data to the first radio apparatus 10 a (S140). Assumehere that a communication has already been executed between the firstradio apparatus 10 a and the second radio apparatus 10 b and the datarate has been set to a predetermined value. The first radio apparatus 10a derives receiving weight vectors, based on the received data (S142).The first radio apparatus 10 a derives transmission weight vectors,based on the estimated receiving weight vectors and then sets these(S144). The first radio apparatus 10 a performs a receiving processingon the received data. While carrying out beamforming by the derivedtransmission weight vectors, the first radio apparatus 10 a transmits tothe second radio apparatus 10 b the rate request signal and trainingsignals, as illustrated in FIG. 3B (S146).

Based on the training signals, the second radio apparatus 10 b carriesout channel estimation (S148). Based on the estimated channel, thesecond radio apparatus 10 b updates rate information (S150). The secondradio apparatus 10 b derives transmission weight vectors and sets these(S152). The second radio apparatus 10 b transmits the rate informationto the first radio apparatus 10 a (S154). In so doing, the adaptivearray signal processing is carried out by using the transmission weightvectors. The first radio apparatus 10 a sets receiving weight vectors,based on a burst signal that contains the rate information (S156). Then,while using the receiving weight vectors, the rate information undergoesa receiving processing (S158). The first radio apparatus 10 a resets thedata rate by referring to the accepted rate information (S160)

FIG. 16 is a flowchart showing still another procedure of setting a datarate in the first radio apparatus 10 a. FIG. 16 corresponds to theoperation of first radio apparatus 10 a shown in FIG. 15. The radio unit20 receives data (S170) The processing unit 22 computes receiving weightvectors (S172) and sets transmission weight vectors (S174). While itcarries out beamforming by the transmission weight vectors in a form aof training signals as shown in FIG. 3B, the processing unit 22transmits the training signals from antenna 12 and, at the same time,transmits a rate request signal (S176).

If the IF unit 26 does not accept rate information via the antenna 12,radio unit 20, processing unit 22 and modem unit 24 (N of S178), keepwaiting until the IF unit 26 accepts it. If, on the other hand, the IFunit 26 accepts the rate information (Y of S178), the processing unit 22sets the receiving weight vectors (S180). The processing unit 22, modemunit 24 and IF unit 26 each carries out receiving processing (S182). Thecontrol unit 30 sets a data rate (S184). The rate information managingunit 32 stores the rate information.

Next, a description on the generation of rate information will be given.The generation of rate information is done in Step 44 of FIG. 9. It isdone by the second radio apparatus 10 b. When the direction in which therate request signal is transmitted is one from the second radioapparatus 10 b to the first radio apparatus 10 a, the rate informationis also generated by the first radio apparatus 10 a. However, thegeneration of rate information will be described herein as theprocessing to be carried out by the second radio apparatus 10 b. In thiscase, the structure shown in FIG. 6 is replaced by that with the antenna14 instead of the antenna 12. FIG. 17 illustrates a structure of acontrol unit 30. The control unit 30 includes a correlation computingunit 70, a power ratio computing unit 72, a processing-objectdetermining unit 74, a rate determining unit 76 and a storage 78.

The processing performed by the control unit 30 is based on theassumption, as described earlier, that the radio unit 20, processingunit 22 and modem unit 24 shown in FIG. 6 all receive trainings signalvia the antenna 14. As shown FIG. 3B, the training signals aretransmitted from a plurality of antennas 12 that include antennas otherthan the first antenna 12 a and second antenna 12 b to transmit thefirst data and second data. The training signal corresponds to“MIMO-LTS”. The respective training signals are so defined as tocorrespond respectively to a plurality of antennas 12. Based on thereceived training signals, the receiving weight vector computing unit 54computes receiving weight vectors corresponding respectively to theplurality of antennas 12. A method for computing the receiving responsevectors is implemented as described above and the repeated descriptionthereof is omitted here. The OFDM modulation scheme is applied to thetraining signals received, as described above, and a plurality ofsubcarriers are used. Hence, the receiving response vectors arecalculated for a plurality of subcarriers, respectively.

The correlation computing unit 70 computes, from the receiving responsevectors, correlations among the receiving response vectors correspondingrespectively to a plurality of antennas 12. Although the channelcharacteristics, namely, the receiving response vectors, correspondingto the first antenna 12 a are denoted as “h₁₁”, “h₁₂”, “h₁₃” and “h₁₄”in FIG. 1, these are brought together and generically called “h₁” hereand it is assumed here that the number of antennas 12 is “2”. If assumedaccordingly, then the correlation computing unit 70 computes acorrelation value S which is expressed by the following Equation (5).

$\begin{matrix}{S = \frac{h_{1}^{H}h_{2}}{\sqrt{h_{1}^{H}h_{1}}\sqrt{h_{2}^{H}h_{2}}}} & (5)\end{matrix}$

The thus computed correlation value S is the value corresponding to onesubcarrier, and the correlation computing unit 70 derives correlationvalues S, respectively, that correspond to a plurality of subcarriers.It is to be noted here that the numerator in Equation (5) may serve asthe correlation value S.

The power ratio computing unit 72 computes, from the receiving responsevectors, power ratios among the receiving response vectors correspondingrespectively to a plurality of antennas. The power ratio computing unit72 computes a power ratio R which is expressed by the following Equation(6).

$\begin{matrix}{R = \frac{h_{1}^{H}h_{1}}{h_{2}^{H}h_{2}}} & (6)\end{matrix}$

The thus computed power ratio R is the value corresponding to onesubcarrier, and the power ratio computing unit 72 derives power ratios,respectively, that correspond to a plurality of subcassiers.

The processing-object determining unit 74 inputs a plurality ofcorrelation values S and power ratios R corresponding respectively to aplurality of subcarriers. The processing-object determining unit 74determines an object to be used to determine a data rate, from aplurality of correlation values S and a plurality of power ratios. Oneof methods for determining the object is to select a correlation value Sand power ratio R that correspond to any of the plurality ofsubcarriers. For example, a measuring unit, which is not shown here,measures the signal strength of the respective subcarriers and theprocessing-object determining unit 74 selects a subcarrier whose signalstrength is large. Alternatively, a statistical processing, such astaking the average, is performed on a plurality of correlation values Sand a plurality of power ratios R, and derives the correlation values Sthat have undergone the statistics processing and the power ratios Rthat have undergone the statistics processing. Hereinafter, thecorrelation values S and power ratios R which have been determined bythe processing-object determining unit 74 will be also referred to asthe correlation value S and power ratio R.

Based on the correlation value S and the power ratio R from theprocessing-object determining unit 74, the rate determining unit 76determines a data rate for data. In so doing, criteria stored in thestorage 78 are referred to. FIG. 18 illustrates a structure of criteriastored in the storage 78. The criteria are so defined as to form atwo-dimensional space by the correlation values and the power ratios,and the two-dimensional space is divided into a plurality of partialregions, namely, “A”, “B”, “C” and “D” as shown in FIG. 18. Here, theplurality of partial regions constituted by the regions “A”, “B”, “C”and “D” each corresponds to a predetermined data rate. For example, ifthe partial regions are corresponded to the number of antennas 12, theregion “A” corresponds to “4” antennas, “B” to “3”, “C” to “2” and “D”to “1”.

It is to be noted that the modulation scheme and the coding rate may bedefined in the similar manner, too, and by using this added combinationthereof the two-dimensional space may be further divided into an addedplurality of partial regions. Referring back to FIG. 17, the ratedetermining unit 76 associates an inputted correlation value S and powerratio R with a criterion and then identifies a partial region thatcontains the inputted correlation value S and power ratio R. Then therate determining unit 76 derives a predefined data rate from theidentified partial region. Upon acceptance of a rate request signal, thecontrol unit 30 carries out the aforementioned processing. When the rateinformation is transmitted, the determined data rate is included in thisrate information. The rate determining unit 76 may determine a data ratefor data, based on either the correlation value S or power ratio R. Insuch a case, the processing can be simplified.

Next, a burst format which is modified over the burst format shown inFIG. 3 b will be explained. As shown in FIG. 3B, training signals aretransmitted from a plurality of antennas 12 in order for the secondradio apparatus 10 b to estimate a plurality of channels. As describedearlier, a part such as “First MIMO-STS” is used to set the gain of AGCwhile a part such as “First MIMO-LTS” is used to estimate channels. Withthe structure as shown in FIG. 3B under the following situation, thereceiving characteristics of the first data and second data possiblysuffer deterioration. If the propagation loss in the channel fromantennas in which no data is being transmitted, namely, the thirdantenna 12 c and the fourth antenna 12 d, is smaller than thepropagation loss in the channel from the other antennas, the receivingstrength at the second radio apparatus 10 b gets large to some extentdue to the “Third MIMO-STS” and “Fourth MIMO-STS”. For such occasions,the gain of AGC is set to a low value. As a result, when the “FirstData” and “Second Data” are demodulated, the gain is not in the enoughlevel, so that the error is likely to occur. A description will be givenhere of a burst format by which to suppress such deterioration ofchannel quality. The burst format is formed in the processing unit 22,based on an instruction from the control unit 30.

FIGS. 19A and 19B illustrate another structures of burst format in thecommunication system 100. FIG. 19A corresponds to a case when threeMIMO-LTSs are allotted respectively to three antennas 12 and two Dataare allotted respective to two antennas 12. Components therein from“Legacy-STS” to “MIMO signal” are the same as those shown in FIG. 3B,and the description thereof is omitted here. “MIMO-LTSs” are allocatedrespectively to the three antennas 12 that include antennas other thanthose used to transmit “MIMO-STS”. That is, the number of antennas 12that should transmit “MIMO-LTSs” is determined based on the number ofchannels to be estimated. On the other hand, the number of antennas 12that should transmit “MIMO-STSs” is made equal to the antennas 12 thatshould transmit “Data”. In other words, two sets of “MIMO-STS” and“Data” are defined at a time, and those are allotted respectively to thesame two antennas 12. Thus, at the setting of the gain of AGC, thesignal strength at the instant when “Data” is received is brought closeto that at the instant when “MIMO-STS” is received. As a result, thedeterioration of receiving quality due to the gain of AGC can beprevented.

In the burst format of FIG. 19A, “MIMO-STS” is transmitted from theantenna 12. Here, “First MIMO-STS” and “Second MIMO-STS” are so definedas to use different subcarriers from each other. For example, “FirstMIMO-STS” uses the odd-numbered subcarriers while “Second MIMO-STS” usesthe even-numbered subcarriers. The relation between these two MIMO-STSsin terms of such the use of subcarriers is called “tone interleaving”.The tone interleaving is carried out in “MIMO-LTSs” among three antennas12. When “first MIMO-LTS” and the like are subjected to thetone-interleaving, the number of OFDM symbols are extended to threetimes the original number thereof, compared with when thetone-interleaving is not executed.

In FIG. 19B, two MIMO-LTSs are allocated respectively to two antennas.This corresponds to a case where one data is allocated to one antenna12. As described earlier, if the number of “Data” is one, “MIMO-STS” canbe put to a common use with “Legacy STS”. Since “Legacy STS” is a signalnecessary for maintaining compatibility with a communication systemwhich is not compatible with a MIMO system, “Legacy STS” cannot beomitted. Thus, “MIMO-STS” is omitted. Accordingly, “Legacy STS” may alsobe said to correspond to “MIMO-STS”.

FIG. 20 illustrates still another structure of burst format in thecommunication system 100. Similar to FIG. 19A, FIG. 20 corresponds to acase when three “MIMO-LTSs” are allocated respectively to three antennas12 and two data are allocated respectively to two antennas 12. As for“MIMO-LTSs”, the same as with FIG. 19A holds true. The control unit 30increases the number of antennas 12 that should transmit “MIMO-STS” upto the number of antennas that should transmit “MIMO-LTS”. In otherwords, as shown in FIG. 20, the number of antennas 12 is increased from“2” of FIG. 19A to “3” of FIG. 20. Furthermore, the data correspondingrespective to the antennas 12 the number of which is not yet increasedare segmented, and the thus segmented data are associated to theantennas 12 the number of which has been increased.

The data corresponding respective to the antennas 12, the number ofwhich corresponds to that prior to increasing the number of antennas,corresponds to, for example, “Second Data” of FIG. 19. The control unit30 segments the “Second Data” into “First-half Data” and “Second-halfData” as shown in FIG. 20. When the data is segmented, the control unit30 carries out the data segmentation on a subcarrier-by-subcarrierbasis. That is, “First-half Data” and “Second-half Data” are in atone-interleave relationship. In this case, too, at the setting of thegain of AGC, the signal strength at the instant when “Data” is receivedis brought close to that at the instant when “MIMO-STS” is received. Asa result, the deterioration of receiving quality due to the gain of AGCcan be prevented.

FIGS. 21A to 21D illustrate still another structure of burst format inthe communication system 100. Similar to FIG. 19A, FIGS. 21A to 21D alsocorrespond to a case when three “MIMO-LTSs” are allocated respectivelyto three antennas 12 and two data are allocated respectively to twoantennas 12. As for “MIMO-STSs” and “Data”, the same as with FIG. 19Aholds true. The control unit 30 assigns a part, in “MMO-LTS”,corresponding to antennas 12 to transmit “MIMO-STS” and a part, in the“MIMO-LTS”, corresponding to antenna 12 other than the antennas 12 totransmit the “MIMO-STS” in such a manner as to have different timings.Here, the antennas 12 to transmit the “MIMO-STS” are the first antenna12 a and the second antenna 12 b.

Accordingly, the parts corresponding thereto correspond to “FirstMIMO-LTS” and “Second MIMO-LTS”. On the other hand, the antenna 12 otherthan the antennas 12 to transmit the “MIMO-STS” is the third antenna 12c, and the part corresponding thereto corresponds to “Third MIMO-LTS”.As shown in FIGS. 21A to 21D, these formats are so assigned that timingsthereof are varied or shifted. It is to be noted that “Third MIMO-LTS”is so defined as to use all of subcarriers. According to such formats asthese, when “First MIMO-LTS” and “Second MIMO-LTS” are amplified by AGC,“Third MIMO-LTS” has no effect on them, so that the channel estimationby use of these formats can be made more accurately. In this case, too,at the setting of the gain of AGC, the signal strength at the instantwhen “Data” is received is brought close to that at the instant when“MIMO-STS” is received. As a result, the deterioration of receivingquality due to the gain of AGC can be prevented.

FIG. 21B corresponds to a case when two MIMO-LTSs are allottedrespectively to two antennas 12 while one data is allotted to oneantenna 12. As shown in FIG. 21B, the structure of burst format shown inFIG. 21B corresponds to that shown in FIG. 21A. The same is true forFIG. 21C but “First MIMO-STS” is omitted in FIG. 21C. It can also besaid that “Legacy STS” corresponds to “MIMO-STS”. FIG. 21D is in thesame situation as with FIG. 21B but “MIMO Signal” is further omitted.Thus, the overhead in a burst signal can be made small. In such a case,a control signal for a MIMO system is not contained therein, so that itis necessary to become aware beforehand that the burst signal inquestion has been transmitted. For example, a training request signalhas been transmitted in advance.

Various modifications to the burst format shown in FIG. 20 will bedescribed hereinbelow. In the burst format show in FIG. 20, the numberof MIMO-STSs, MIMO-LTSs and Data are the same. That is, the MIMO-STS,MIMO-LTS and Data are transmitted from three antennas 12, respectively.According to such a burst format as shown in FIG. 20, the number ofMIMO-STS is equal to that of Data, so that the error contained in thesetting of AGC at the receiving side is reduced when a receivingprocessing is carried out for Data. Furthermore, the MIMO-LTS istransmitted from a plurality of antennas 12, so that it is possible toestimate the channels corresponding to the plurality of antennas 12 atthe receiving side. Furthermore, the number of MIMO-STSs is equal tothat of MIMO-LTSs, so that the degree of accuracy in channel estimationbased on MIMO-LTS is raised.

In a modification described below, the following advantageous featuresare added. For instance, assume now that the number of antennas 12 is“3” and the data to be transmitted is composed of “2” streams. The dataare turned into “3” streams by segmenting any of “two”-stream data, andthen the data are allotted respectively to “3” antennas 12. In such acase, there exist a plurality of combinations of segmentation of dataand allotment of antennas 12. When the number of antennas 12 increases,the number of combinations also increases. In other words, if any ofdata streams is segmented and allotted to any of antennas 12, theprocessing may possibly become complicated. It is an object of a firstmodification to reduce the processing amount or throughput and, at thesame time, allot the data to the antennas 12 even if the number of datastreams to be transmitted is less than the number of antennas 12.

FIGS. 22A and 22B illustrate structures of burst format modified overthat of FIG. 20, and these correspond to the first modification. Similarto what has been described above, in each of FIGS. 22A and 22B, the toprow indicates a signal corresponding to the first antenna 12 a; themiddle row a signal corresponding to the second antenna 12 b; and thebottom row a signal corresponding to the third antenna 12 c. On oneoccasion these are collectively called a burst signal, whereas onanother occasion a signal transmitted from one antenna 12 is called theburst signal. In this patent specification, the term “burst signal” willbe used without any such distinction. “MIMO-LTS” and the like serving asknown signals and Data are contained in the burst signal. In FIG. 22A,Legacy STS (hereinafter referred to as “L-STS”), Legacy LTS (hereinafterreferred to as “L-LTS”), Legacy signal (hereinafter referred to as“L-signal”) and MIMO signal are allotted to the first antenna 12 a only.

A structure subsequent to the above is as follows. In the followingdescription, data is assumed to correspond to two antennas 12. That is,it is assumed herein that the number of data streams is less than thenumber of antennas 12. The control unit 30 as shown in FIG. 6 increasesthe number of antennas 12 that should transmit MIMO-STS and Data, up tothe number of antennas 12 that should transmit MIMO-LTS. That is, if theData corresponds to at least one of the antennas 12 that should transmitMIMO-LTS, the control unit 30 associates the Data to antenna 12 thatshould transmit MIMO-LTS by increasing the number of antennas 12 to beassociated thereto. Since the number of antennas 12 that should transmitMIMO-LTS is “3” here, the number of antennas 12 that should transmitMIMO-STS and Data becomes “3”, too. The data, which correspondrespectively to antennas 12 prior to increasing the number thereof,namely, the “2”-stream data, are segmented and the thus segmented dataare associated respectively to antennas 12 having the number of antennas12 that should transmit MIMO-LTS.

Describing the above more specifically, the control unit 30 causes theIF unit 26 to combine the “2”-stream data into one, segment the thuscombined data into “3” data and allot the “3” data to “3” antennas 12.The data may be assumed to correspond to “2” streams and then treated asone data. In this case, the data are not combined but segmented into“3”. The number of antennas 12 may be other than “3”. Here, for example,the segmentation of data are so carried out as to be of approximatelyevenly divided data amount for each of a plurality of antennas 12. Also,the data may be segmented according to a predefined rule. As a result ofthe above processing, MIMO-STS, MIMO-LTS and Data are allottedrespectively to the three antennas 12, as shown in FIG. 22A. In FIG. 22Aor FIG. 22B, the data are represented as “first segmented data”, “secondsegmented data” and “third segmented data”.

As described earlier, the control unit 30 uses a plurality ofsubcarriers for MIMO-LTS and Data, and varies the combination ofsubcarriers to be used respectively for MIMO-LTSs, for each of theplurality of antennas 12. In other words, the MIMO-LTSs correspondingrespectively to the first antennas 12 a to the third antennas 12 usedifferent subcarriers, respectively. In FIG. 22A, a first MIMO-LTS(1)uses ⅓ of entire subcarriers, the second MIMO-LTS(1) uses also ⅓ ofentire subcarriers, and the third MIMO-LTS(1) uses also ⅓ of entiresubcarriers. It is assumed that the subcarrier used for the firstMIMO-LTS(1) to third MIMO-LTS(1) do no overlap. The same relationshipholds among first MIMO-LTS(2) to third MIMO-LTS(2). The samerelationship also holds among first MIMO-LTS(3) to third MIMO-LTS(3).Also, the first MIMO-LTS(1), the first MIMO-LTS(2) and the firstMIMO-LTS(3) use mutually different subcarriers. The first MIMO-LTS(1),the first MIMO-LTS(2) and the first MIMO-LTS(3) are MIMO-LTSs allottedto different symbols.

The above rule can be interpreted as follows. For the period of onesymbol, MIMO-LTSs allotted respectively to a plurality of antennas 12use mutually different subcarriers. While MIMO-LTSs, which are allottedto one antennas 12 and contained over a plurality of symbols, are usingmutually different subcarriers, respectively, they use, as a whole, allof subcarriers to be used. When data are associated to antennas 12 thatshould transmit MIMO-LTS, a combination of subacarriers in MIMO-LTStransmitted from the same antennas 12 as the one from which the data aresent is used for the data in question. For example, it is structuredsuch that subcarriers used for the first segmented data are the same asthose used for the first MIMO-LTS(1). By carrying out such a processing,the processing amount necessary for segmenting data is reduced and atthe same time the data can be allotted respectively to a plurality ofantennas 12.

The subcarries used for MIMO-LTS placed in the beginning are the same asthose used for Data. Hence, when a receiving apparatus, not shown,receives MIMO-LTSs assigned in a plurality of symbols, the channelestimation is carried out from at least the MIMO-LTS placed in thebeginning, and the Data are demodulated based on the result thereof.Subcarriers used for MIMO-LTS other than the MIMO-LTS placed in thebeginning differ from those used for Data. Hence, even if suchsubacarriers are not used for demodulation, the deterioration in thequality of demodulation is suppressed. Thus, the receiving apparatus mayskip a processing for MIMO-LTSs other than the MIMO-LTS placed in thebeginning. As a result, the processing amount can be reduced and thesame processing as with a receiving apparatus compliant with theIEEE802.11a standard can be applied.

FIG. 22B illustrates a modification of the burst format shown in FIG.22A. MIMO-STS and its subsequent portions in FIG. 22B are the same aswith FIG. 22A. L-STS to MIMO Signal are allotted to the second antenna12 b and the third antenna 12 c, too. In this case, for example, CDD(Cyclic Delay Diversity) is performed on L-STSs allotted to the secondantenna 12 b and the third antenna 12 c. That is, the L-STS allotted tothe second antenna 12 b undergoes a timing shift against the L-STSallotted to the first antenna 12 a. The same is applied to the L-STSallotted to the third antenna 12 c.

FIG. 23 is a flowchart showing a transmission procedure corresponding tothe burst formats shown in FIGS. 22A and 22B. If the transmission oftraining signals is necessary (Y of S220) and the number of antennas 12that should transmit data is less than the number of antennas 12 thatshould transmit the training signals (Y of S222), the control unit 30acquires subcarriers corresponding to the training signals for each of aplurality of antennas 12 (S224). While using the thus acquiredsubcarriers, the control unit 30 associates the data with the antennas12 that should transmit the training signals (S226). That is, similar tothe training signals, the data corresponding to a plurality of antennas12 are mutually subjected to tone interleaving.

The control unit 30 generates a burst signal from at least the trainingsignals and data (S228). If, on the other hand, the number of antennas12 that should transmit data is not less than the number of antennas 12that should transmit the training signals (N of S222), namely, if thenumber of antennas 12 that should transmit data is equal to the numberof antennas 12 that should transmit the training signals, the controlunit 30 generates a burst signal from at least the training signals anddata (S228). The radio apparatus 10 transmits the burst signal (S230).If the transmission of training signals is not necessary (N of S220),the processing is terminated.

Next, a second modification of burst format shown in FIG. 20 will bedescribed. If the characteristics of channel from any of a plurality ofantennas is not suited for the transmission of data when MIMO-LTS istransmitted from the plurality of antennas 12, there is a possibilitythat data transmitted from the plurality of antennas 12 is corrupted andthus erroneous. It is an object of the second modification to reduce theprobability of the data error even if the data are transmitted from aplurality of antennas. The burst format according to this secondmodification is represented by FIGS. 22A and 22B.

If data corresponds to at least one of antennas 12 that should transmitMIMO-LTS, the control unit 30 of FIG. 6 has the data associated to theantennas 12 that should transmit MIMO-LTS by increasing the number ofantennas to be associated thereto, as described earlier. The controlunit 30 determines the data rate of data contained in the burst signal.If the data are associated to the antennas 12 that should transmitMIMO-LTS, the control unit 30 decides on a data rate which is lower thanthe data rate set before the data is associated thereto, for theantennas 12 that should transmit MIMO-LTS. For example, assume that thenumber of antennas 12 that should transmit MIMO-LTS is “3” and the datais composed of “2” streams. If the data rate of “2”-stream data is 100Mbps, the data rate when the data is turned into “3”-stream data will be50 Mbps. “The data rate set before the data is associated thereto, forthe antennas 12 that should transmit MIMO-LTS” may be the data ratewhich has been used for a communication so far, or may be the data ratedetermined according to a channel characteristics. Here, as describedearlier, the data rate is determined by the modulation scheme, codingrate of error correction and the number of antennas 12.

FIG. 24 is a flowchart showing another transmission procedurecorresponding to the burst formats shown in FIGS. 22A and 22B. If thetransmission of training signals is necessary (Y of S200) and the numberof antennas 12 that should transmit data is less than the number ofantennas 12 that should transmit the training signals (Y of S202), thecontrol unit 30 has the data associated to the antennas that shouldtransmit the training signals (S204). The control unit 30 lowers thedata rate for data (S206). The control unit 30 generate a burst signalfrom at least the training signals and data (S208). If, on the otherhand, the number of antennas 12 that should transmit data is not lessthan the number of antennas 12 that should transmit the training signals(N of S202), namely, if the number of antennas 12 that should transmitdata is equal to the number of antennas 12 that should transmit thetraining signals, the control unit 30 generates a burst signal from atleast the training signals and data (S208). The radio apparatus 10transmits the burst signal (S210). If the transmission of trainingsignals is not necessary (N of S200), the processing is terminated.

According to the first embodiment, when a request signal is transmittedto a targeted radio apparatus, training signals are transmitted from aplurality of antennas. Thus, the rate information, about the targetedradio apparatus, which has been generated based on the training signalscan be obtained and therefore the degree of accuracy in rate informationcan be improved. The rate information is determined in consideration ofthe effect of various channels by using the training signals, so thatthe degree of accuracy in rate information can be improved. Since therequest signal and the training signals are transmitted consecutively,the most recent rate information can be obtained. Since the latestupdated rate information can be acquired, the error in rate informationcan be made small even if a channel fluctuates. Moreover, wheninformation on the data rate of a targeted radio apparatus is needed,the request signal is transmitted. Thus, even if the rate information isnot transmitted on a periodic basis, accurate rate information can beobtained. With the improved accuracy of rate information, the occurrenceof data error is reduced and the accuracy of control in transmittingdata can be improved. Since the rate request signal and the trainingsignals are transmitted in a combined manner, the deterioration ofeffective data rate can be prevented.

Since the number of antennas that should transmit training signals isreduced, the power consumption can be reduced. The antennas to be usedfor a communication transmit the training signals, so that thedeterioration of characteristics can be suppressed. Since the powerconsumption can be reduced, the operable period can be extended even ifthe radio apparatus is powered by a battery. Since the power consumptioncan be reduced, the radio apparatus can be made smaller in size. Sinceantennas that have higher signal strength are preferentially selected,the deterioration of quality in data transmission can be prevented.Since antennas are selected according to the wireless quality, thedeterioration of quality in data transmission can be prevented whilereducing the power consumption. The known signals are transmitted fromantennas that should transmit data, so that the deterioration oftransmission weight vectors derived in a radio apparatus to becommunicated is prevented. Also, antennas that should transmit data areselected, so that the power consumption is reduced. Since the derivedtransmission weight vectors are accurately produced, the deteriorationof antenna directivity can be prevented.

The beamforming is carried out at the time of transmitting the trainingsignals, so that the signal strength at a targeted radio apparatus canbe raised and the information on a rate having faster values can beobtained. The beamforming is also executed at the time of actuallytransmitting the data, so that a data rate suited for the occasion of adata rate transmission can be obtained. When a data rate is determined,the values of correlation among the receiving response vectors and theratios in strength among receiving response vectors are taken intoaccount, so that the effect among signals transmitted respectively froma plurality of antennas can be reflected. The degree of accuracy of thedetermined rate information can be improved. In a MIMO system, when thecorrelation value becomes smaller, the channel characteristics improve.Also, when the strength ratio becomes smaller, they improve. Thus, thedata rate can be so determined as to reflect such characteristics. Thedecision based on the correlation values and strength ratios can beapplied to a system in which a plurality of carriers are used. When thetraining signals are received, the rate request signal is also received.Thus, the rate information determined can be notified and the highlyaccurate rate information can be provided.

Since the antennas for transmitting MIMO-STSs are the same as those fortransmitting Data, the signal strengths at the time when MIMO-STS isreceived and Data is received in setting AGC gain at a receiving sidecan be brought close to each other. The deterioration of receivingquality due to AGC gain can be prevented. The effect of portionscorresponding to antennas other than antennas to transmitting MIMO-STSupon those corresponding to antennas to transmit MIMO-STS can be madesmaller, so that the accuracy of channel estimation at portionscorresponding to the antennas to transmit MIMO-STS can be improved at areceiving side. The interference among the segmented data can be madesmall.

Even if, in a case where data are associated to antennas that shouldtransmit MIMO-LTS, the characteristics of radio channel from the thusassociated antenna are not suited for data transmission, the occurrenceof data error can be reduced by lowering the data rate. When the numberof antennas transmitting MIMO-LTS is increased, the number of antennastransmitting MIMO-STS can be increased, too, and the data streams havingthe same number of streams as the number of antennas transmittingMIMO-STS can be transmitted. In a case when the number of data streamsis increased, the deterioration of transmission quality of data can beprevented. In a case where data are associated to a plurality ofantennas, the same subcarriers are used for MIMO-LTS and Datacorresponding to each antenna, so that the selection of subcarriers tobe used for the respective data can be facilitated. Even in cases wherethe number of antennas that should transmit MIMO-LTSs and the number ofdata streams vary, the allotment of data to antennas can be facilitated.If two modifications to FIG. 20 are combined, the advantageous aspectsgained by both the two modifications can be obtained.

Second Embodiment

Similar to the first embodiment, a second embodiment of the presentinvention relates to a MIMO system and it particularly relates to atransmitting apparatus in the MIMO system. The transmitting apparatusaccording to the present embodiment corresponds to transmittingfunctions in the first radio apparatus or second radio apparatus in thefirst embodiment. In the same situation as in the first embodiment wherethe training signals are to be transmitted, the transmitting apparatustransmits training signals. Here, a description will be given centeringaround a burst format containing training signals, and the repeateddescription on the same situation, in which the training signals are tobe transmitted, as in the first embodiment is omitted. The transmittingapparatus transmits a burst signal composed of a plurality of streams,namely a burst signal of multiple streams, corresponding to a pluralityof antennas and assigns a plurality of MIMO-STSs in a burst signalcomposed of a plurality of streams. Subsequent to a plurality ofMIMO-STSs, the transmitting apparatus assigns a plurality of MIMO-LTSsin the burst signal composed of a plurality of streams. The transmittingapparatus also assigns Data in part of the burst signal composed of aplurality of streams. The transmitting apparatus increases data up tothe number of a plurality of streams by multiplying the data by asteering matrix. The transmitting apparatus also multiplies MIMO-LTS bya steering matrix. However, the transmitting apparatus does not multipleMIMO-STS by a steering matrix. In what is to follow, a burst signal of aplurality of streams that has been multiplied by a steering matrix willbe called “a burst signal of a plurality of streams” (namely, “a burstsignal of multiple streams”) also as before without distinguishingtherebetween.

Here, MIMO-STS has a predetermined cycle. More specifically, a guardinterval is added to a signal having a cycle of 1.6 μs. Theaforementioned steering matrix contains therein a component in which atime shifting is cyclically executed for each stream. The cyclicallyexecuted time shift is the so-called CDD (Cyclic Delay Diversity). Here,the cyclic time shifting is performed on a cycle of pattern contained inMIMO-LTS. The similar processing is performed on Data as well. Althoughtime-shift amounts differ for each burst signal of a plurality ofstreams, at least one of these time-shift amounts is set to an amountgreater than or equal to the cycle in MIMO-STS. According to theprocessing described as above, the transmitting apparatus deforms aburst signal of a plurality of streams and transmits the burst signal ofa plurality of deformed streams from a plurality of antennas,respectively.

Problems associated with the above embodiments may be expressed asfollows. Want to transmit the training signals by such a burst format asto improve the accuracy of channel estimation in a targeted radioapparatus. Want to transmit the training signals by such a burst formatas to improve the accuracy of rate information in a targeted radioapparatus. Want to transmit data by such a burst format as to preventthe deterioration of communication quality of data even in a case whenthese training signals are transmitted. Want to utilize the trainingsignals to have the data received.

FIG. 25 illustrates a structure of a transmitting apparatus 300according to a second embodiment of the present invention. Thetransmitting apparatus 300 includes an error correcting unit 310, aninterleaving unit 312, a modulation unit 314, a preamble adding unit316, a spatial spreading unit 318, a first radio unit 20 a, a secondradio unit 20 b, a third radio unit 20 c and a fourth radio unit 20 d,which are generically referred to as “radio unit 20”, and a firstantenna 12 a, a second antenna 12 b, a third antenna 12 c and a fourthantenna 12 d, which are generically referred to as “antennas 12”. Thetransmitting apparatus 300 corresponds to part of the first radioapparatus 10 a shown in FIG. 6.

The error correcting unit 310 carries out coding for error correction.Here, the convolutional coding is carried out and the coding ratethereof is selected from among predefined values. The interleaving unit312 interleaves data on which the convolutional coding has beenperformed. The interleaving unit 312 separates data into a plurality ofstreams before outputting the data. Here, suppose that the data areseparated into two streams. The data of two streams are mutuallyindependent from each other.

The modulation unit 314 modulates the data of two streams, respectively.The preamble adding unit 316 adds a preamble to the modulated data. Forthat purpose, the preamble adding unit 316 stores MIMO-STSs, MIMO-LTSsand so forth as preambles. The preamble adding unit 316 generates aburst signal, composed of a plurality of streams, that containsMIMO-STSs and MIMO-LTSs assigned respectively in a plurality of streamsand Data assigned in at least one of the plurality of streams. Asdescribed earlier, the data is formed by two streams. It is assumedherein that a plurality of streams is “4”. Thus, MIMO-STSs and MIMO-LTSsare assigned respectively in a burst signal of four streams, and Dataare assigned in two of four streams in the burst signal of four streams.As a result, a burst signal of four streams is outputted from thepreamble adding unit 316.

Though the detailed description of MIMO-STSs is omitted here, STSscorresponding to at least one of a plurality of streams in a burstsignal of a plurality of streams may, for example, be defined to usesubcarriers at least part of which differs from those corresponding to aburst signal of other streams. STSs may be defined in a manner such thatthe number of subcarriers to be used for each STS is the same andmutually different subacarriers are used. As described earlier, burstsignals of a plurality of streams use different subcarrries,respectively, and MIMO-LTSs assigned in the burst signals of a pluralityof streams use different subcarries for each stream. In other words, thetone interleaving is carried out. Each of burst signals of a pluralityof streams may be called “burst signal”. Also, the burst signals of aplurality of streams may be collectively called “burst signals”. In thispatent specification, the term “burst signal” will be used without anysuch distinction therebetween.

The spatial spreading unit 318 multiplies, by a steering matrix each,the MIMO-LTS and the data among the burst signals of a plurality ofstreams so as to generate the MIMO-LTS multiplied by the steering matrixand the data whose count has been increased to the number of a pluralityof streams. Before the multiplication, the spatial spreading unit 318extends the degree of inputted data up to the number of a plurality ofstreams. The number of inputted data is “2” and is represented here by“Nin”. Hence, the inputted data is expressed by a vector “Nin×1”. Thenumber of a plurality of streams is “4” and is represented here by“Nout”. The spatial spreading unit 318 extends the degree of inputteddata from Nin to Nout. That is, the vector “Nin×1” is extended to avector “Nout×1”. In so doing, “0's” are inserted to components from(Nin+1)th row to Nout-th row.

A steering matrix S is expressed by the following Equation (7).

S(l)=C(l)W  (7)

The steering matrix is a matrix of “Nout×Nout”. W is an orthogonalmatrix. One example of the orthogonal matrices is Walsh matrix. Here,“l” indicates the subcarrier number, and the multiplication by thesteering matrix is carried out on a subcarrier-by-subcarrier basis. C isexpressed by the following Equation (8) and is used for CDD.

C(f)=dia(1,exp(−j2πlδ/Nout), . . . , exp(−j2πlδ(Nout−1)/Nout))  (8)

In Equation (8), δ indicates a shift amount. That is, the spatialspreading unit 318 carries out, stream by stream, the cyclic timeshifting in the MIMO-LTS multiplied by an orthogonal, by a shift amountcorresponding to each of a plurality of streams, and at the same timecarries out, stream by stream, the cyclic time shifting in the datawhose count has been increased to the number of a plurality of streams.It is to be noted that the structure of MIMO-LTS is similar to that ofLegacy LTS which is equivalent to LTS in the IEEE802.11a standard. Theshift amount is set to a value that differs for each stream. In thesetting of shift amounts, at least one of shift amounts correspondingrespectively to a plurality of streams is so set as to be greater thanor equal to a predetermined cycle that the MIMO-STS had. Since the cyclethat the MIMO-STS had is 1.6 μs, at least one of shift amounts is set to1.6 μs, for instance. In this case, performing time shifting on MIMO-STSis equivalent to the fact that no time shift is generated. Thus, thetime shifting is not performed here on MIMO-STS. As a result of theabove processing, the spatial spreading unit 318 varies, modifies ordeforms the structure of burst signals of a plurality of streams.

There are provided radio units 20 the number of which is equal to thenumber of antennas 12. The radio unit 20 transmits the deformed burstsignals of a plurality of streams. Then the radio unit 20 transmits theburst signals of a plurality of streams by associating them to aplurality of antennas 12. The radio unit 20 includes an IFFT unit, a GIunit, a quadrature modulation unit, a frequency conversion unit and anamplification unit, which are all not shown here. The IFFT unit performsIFFT (Inverse Fast Fourier Transform), thereby converting afrequency-domain signal using a plurality of subcarriers into atime-domain signal. The GI unit adds a guard interval to time-domaindata. The quadrature modulation unit carries out quadrature modulation.The frequency conversion unit performs a frequency conversion bytransforming a quadrature-modulated signal into a radio-frequencysignal. The amplification unit is a power amplifier for amplifyingradio-frequency signals. It is to be noted that the spatial spreadingunit 318 may be provided in a position posterior to the IFFT unit, notshown.

FIGS. 26A and 26B each illustrate a burst format of a burst signalgenerated in the transmitting apparatus 300. FIG. 26A illustrates aburst format of a burst signal of a plurality of streams outputted fromthe preamble adding unit 316. Since FIG. 26A is equivalent to FIG. 3B,the description thereof is omitted. Here, “4” MIMO-STSs and “4”MIMO-LTSs are added to burst signals of a plurality streams, namely,“four” streams, respectively. On the other hand, data of “2” streamswhich are at least one of a plurality of streams are added as “FirstData” and “Second Data”. FIG. 26B illustrates burst signals of aplurality of streams deformed by the spatial spreading unit 318.MIMO-STSs of FIG. 26B are the same as that of FIG. 26A. The MIMO-LTSs ofFIG. 26A are multiplied by a steering matrix so as to become“MIMO-LTS's” of FIG. 26B. In FIG. 26B, these LTSs are indicated as“First MIMO-LTS′” to “Fourth MIMO-LTS′”. As a result of multiplicationby the steering matrix, the “First Data” and “Second Data” are turnedinto Data of four streams. In FIG. 26B, these data are shown as “FirstData′” to “Fourth Data′”.

According to the second embodiment, even if the number of data streamsis less than the number of MIMO-LTS streams, the multiplication byorthogonal matrices and the cyclic time shift processing are carriedout. As a result thereof, the number of data streams can be made equalto the number of MIMO-LTS streams. MIMO-LTS also undergoes the sameprocessing as with Data. Thus, the radio apparatus to be communicatedcan use MIMO-LTS at the time of receiving the Data. MIMO-STS does notundergo the same processing with data streams. Thus, the time shiftamount in CDD can be made large and the receiving characteristics of aradio apparatus to be communicated can be improved. MIMO-LTS istransmitted from all of the antennas, so that an assumed channel can beestimated at a receiving side. Even if the number of data streams is notequal to the number of antennas, signals can be evenly transmitted fromall of the antennas by performing the Walsh matrices and CDD processingon data. The data power can be adjusted to MIMO-LTS.

Since the processing by Walsh matrices and CDD is also performed onMIMO-LTS, the channel estimated by MIMO-LTS can be used intact forreceiving the data at a receiving side. When CDD is performed onMIMO-LTS and Data by a sufficient shift amount, the difference in powerbetween MIMO-LTS and Data becomes very small, so that the accuracy ofAGC setting at a receiving side can be improved. Furthermore, the timeshifting with a large shift amount cannot be performed on MIMO-STS.Hence, in such a case, the power for MIMO-STS and the power for MIMO-LTScan be adjusted by associating MIMO-STS to all of antennas. Also, thepower for MIMO-STS and the power for MIMO-LTS can be adjusted even if noCDD processing is done to MIMO-STS. MIMO-LTS has undergone the toneinterleaving. Hence, even if MIMO-LTS is transmitted from all ofantennas, the transmission power can be retained by the processing ofWalsh matrices and CDD. If the processing of Walsh matrices and CDD isnot carried out and if data of two streams are transmitted by threeantennas, each power within a burst signal is related such that “3STSs”=“3 LTSs”>“2 Data”. If, however, the processing by Walsh matricesand CDD is carried out, it can be related such that “3 STSs”=“3 LTSs”=“3Data”.

Third Embodiment

A third embodiment of the present invention corresponds to an embodimentin which the first embodiment and the second embodiment are combinedtogether. That is, the radio apparatus generates training signals formedby burst formats as in the first embodiment, namely, burst signals of aplurality of streams. The radio apparatus also multiplies the thusgenerated burst signals of a plurality of streams by a steering matrixas in the second embodiment, so as to vary and deform the burst signalsof a plurality of streams. The radio apparatus transmits the thusdeformed burst signals of a plurality of streams from a plurality ofantennas. Here, the radio apparatus may respectively multiply MIMO-STS,MIMO-LTS and Data contained in the burst signals of a plurality ofstreams, by a steering matrix.

A radio apparatus 10 according to the third embodiment is of the sametype as the first radio apparatus 10 a of FIG. 6. The transmittingfunctions in the radio apparatus 10 according to the third embodiment isof the same type as the transmission apparatus 300 of FIG. 25. Thecontrol unit 30 and/or the preamble adding unit 316 generate burstsignals of a plurality of streams which contain MIMO-STS assigned in atleast one of a plurality of streams, MIMO-LTS assigned respectively in aplurality of streams, and Data assigned in the same streams as MIMO-STS.The control unit 30 and/or the preamble adding unit 316 arrange themsuch that a portion assigned in a stream in which MIMO-STS is assigned,among MIMO-LTSs, and a portion assigned in a stream other than thestream assigned in which MIMO-STS is assigned, among MIMO-LTSs, areplaced at different timings. As a result thereof, burst signals havingthe burst formats as shown in FIG. 21A are produced. The spatialspreading unit 318 deforms burst signals of a plurality of streams bymultiplying the thus produced burst signals of a plurality of streams bya steering matrix. The spatial spreading unit 318 also multipliesMIMO-STS by a steering matrix and carries out time shifting with itstime shift amount being an arbitrary value. The rest of operation is thesame as in the second embodiment and the description thereof is omitted.

FIG. 27 illustrates a structure of a burst format according to the thirdembodiment of the present invention. FIG. 27 corresponds to a burstformat for a burst signal of a plurality of streams outputted from thespatial spreading unit 318 and corresponds to a burst format in which asteering matrix is multiplied to the burst format of FIG. 21A. “FirstMIMO-STS′” to “Third MIMO-STS′” of FIG. 27 correspond to a result of“First MIMO-STS” and “Second MIMO-STS” of FIG. 21A multiplied by asteering matrix. The steering matrix corresponds to a “3×3” matrix.Thus, it is extended to “3×1” vectors by adding rows composed of “0's”in “First MIMO-STS” and “Second MIMO-STS” of FIG. 21A. “First MIMO-LTS′”to “Third MIMO-LTS′” of FIG. 27 correspond to a result of “FirstMIMO-LTS” and “Second MIMO-LTS” of FIG. 21A multiplied by a steeringmatrix. “Fourth MIMO-LTS′” to “Sixth MIMO-LTS′” of FIG. 27 correspond toa result of “Third MIMO-LTS” FIG. 21A multiplied by a steering matrix.Data are the same as in the second embodiment.

It is to be noted that burst formats for burst signals of a plurality ofstreams generated by the control unit 30 and/or preamble adding unit 316may correspond to FIGS. 22A and 22B. The description on these burstformats are the same as those in the first embodiment and are thusomitted here. The spatial spreading unit 318 multiples a steering matrixto these burst signals. It is preferred that the burst signals of aplurality of streams, which are the above training signals, aretransmitted at the timing explained in the first embodiment. That is, itis preferred that the control unit 30 generates a rate request signalwith which to supply a radio apparatus 10 with information on a datarate corresponding to a radio channel between radio apparatuses 10, andthe training signals are used when the radio apparatus 10 transmits thethus generated rate request signal. The details thereof are the same asin the first embodiment and the description thereof is omitted here.With a plurality of antennas 12 of the first embodiment replaced by theburst signals of a plurality of streams, the present invention accordingto the first embodiment is applied to the third embodiment. According tothe third embodiment, as described above, a steering matrix ismultiplied to the burst signals of a plurality of streams and thesignals of a plurality of streams multiplied by the steering matrix aretransmitted from a plurality of antennas 12.

According to the third embodiment, when a rate request signal isoutputted to a targeted radio apparatus, MIMO-LTSs assigned in aplurality of streams are outputted while a steering matrix is beingmultiplied thereto. Thus the data rate information, in the targetedradio apparatus, which is newly produced based on MIMO-LTS can beobtained and the accuracy of information can be improved. Various burstformats as shown in the first embodiment can be transmitted from aplurality of antennas by multiplying a steering matrix to the variousburst formats as shown in the first embodiment. Even if the number ofdata is less than that the number of antennas, the data and the like canbe transmitted from a plurality of antennas by multiplying a steeringmatrix thereto. Also, even if the number of data is less than the numberof antennas, the same advantageous effects as when the data and the likeare transmitted from a plurality of antennas can be obtained bymultiplying a steering matrix thereto.

Fourth Embodiment

A fourth embodiment of the present invention relates to a burst formatwhich can be applied to the first to third embodiments. Here, the burstformat may be defined as a burst signal outputted from antennas as inthe first embodiment or defined as a burst signal generated by a controlunit or preamble adding unit as in the second or third embodiment. Aburst format according to the fourth embodiment is used as a burstsignal used when the training signals are transmitted. However, thetiming at which a burst signal having said burst format is to betransmitted may be after the acceptance of training signals or upontransmission of a rate request signal as in the first embodiment.

The radio apparatus 10 according to the fourth embodiment is of the sametype as the first radio apparatus 10 a shown in FIG. 6. Three kinds ofburst formats are explained here. The three kinds of burst formats areeach subdivided into a case where it is defined as a burst signaloutputted from antennas as in the first embodiment and a case where itis defined as a burst signal generated by a control unit and/or preambleadding unit as in the second or third embodiment. They are alsosubdivided in terms of the timing at which the burst signal istransmitted. The burst format per se will be mainly explained here.Specific implementation for such subdivision will be made based on thedescription of first to third embodiments.

The three kinds of burst formats correspond to the case when the numberof antennas 12 that should transmit MIMO-LTS is greater than the numberof antennas 12 that should transmit Data or when the number of streamsin which MIMO-LTS is assigned is greater than the number of streams inwhich Data is assigned. This can be said to be a modification to FIG.3B, FIGS. 19A and 19B, FIGS. 21A to 21D and FIG. 26A.

A first modification will now be described. The first modification isexplained with reference to FIG. 19A. In FIG. 19A, MIMO-LTSs are soassigned as to correspond to the first antenna 12 a to third antenna 12c, and Data are so assigned as to correspond to the first antenna 12 aand second antenna 12 b. The first modification relates to the selectionof antenna 12 for use with Data when the number of antennas 12 thatshould transmit MIMO-LTS (hereinafter, all of or one of such antennas 12will be referred to as “LTS antenna 12” is greater than the number ofantennas 12 that should transmit Data (hereinafter, all of or one ofsuch antennas 12 will be referred to as “Data antenna 12”). Problemsassociated with this modification are expressed as follows. In anantenna 12 where no Data is transmitted, no MIMO-STS is transmitted fromthis antenna 12, too. Accordingly, if the signal strength of MIMO-LTStransmitted from said antenna 12 becomes, at a receiving side, largerthan the signal strength of MIMO-LTS transmitted from the other antenna12, a distortion is likely to occur in received MIMO-LTSs. Hence, Datais likely to suffer an error and the communication quality is likely todeteriorate.

As in Step 64 of FIG. 11, the radio apparatus 10 according to thismodification measures the strength of signals received from a targetedradio apparatus, for each of a plurality of antennas 12. Here, thesignal strength may be measured in not only the MIMO-STS or MIMO-LTSacross burst signals but also an arbitrary portion of the burst signal.Based on the measured signal strength, the radio apparatus 10 selects atleast one antenna 12 from among a plurality of antennas 12. The selectedantenna 12 corresponds to the data antenna 12. For instance, selectedare antennas 12 that have received signals having the largest and thesecond largest signal strength. More specific explanation is as follows.The radio apparatus 10 measures the strength of signals received by thefirst antenna 12 a to third antenna 12 c. The radio apparatus 10 selectsthe first antenna 12 a and second antenna 12 b according to themagnitude of the signal strength measured. A burst signal in which Dataare assigned is produced so as to be associated to the selected firstantenna 12 a and second antenna 12 b. The symmetric property in radiochannel at transmission and receiving sides is utilized here.

In the first modification, the following combination is also possible.The antenna 12 selected for transmitting Data may be contained in theantennas 12 selected for transmitting MIMO-LTS. Accordingly, the firstmodification may be applied to the cases shown in FIG. 11 and FIG. 12.The training signals having the burst format according to the firstmodification may be transmitted after the training request signal hasbeen accepted or when the rate request signal is transmitted, as in thefirst embodiment. Accordingly, the first modification may be applied tothe cases shown in FIG. 4, FIG. 5, and FIG. 9 to FIG. 16. Moreover, amodification may be such that a modified burst format is not so definedas to correspond to the antenna 12 but defined in a burst signalgenerated by the control unit 30 and/or preamble adding unit 316, as inthe second or third embodiment. In such a case, the transmittingfunctions in the radio apparatus 10 may be of the same type as those ofthe transmitting apparatus shown in FIG. 25. A steering matrix may beapplied as in the transmitting apparatus 300 shown in FIG. 25.

A second modification will now be described. The second modificationrelates to MIMO-LTSs transmitted from antennas 12, among LTS antennas12, other than Data antennas 12 when the number of LTS antennas 12 isgreater than the number of Data antenna 12. Problems associated withthis second modification are expressed the same way as those in thefirst modification. In the case of FIG. 19A, the antennas 12, among theLTS antenna 12, other than the Data antennas 12 correspond to the thirdantenna 12 c.

In the second modification, the amplitude of the third MIMO-LTStransmitted from the third antenna 12 c is defined to be a value lessthan the amplitude of the first MIMO-LTS and second MIMO-LTS transmittedfrom the first antenna 12 a and second antenna 12 b, respectively. Thiscorresponds to a case where the amplitude of the third MIMO-LTS is ½ ofthe amplitude in the first MIMO-LTS and the second MIMO-LTS. The firstantenna 12 a and second antenna 12 b correspond to Data antennas 12.According to the second modification, the signal strength of the thirdMIMO-LTS can be made smaller by reducing the amplitude of the thirdMIMO-LTS transmitted from the third antenna 12 c. When the signalstrength of the third MIMO-LTS gets small, the distortion againstMIMO-LTS becomes less likely to occur even if no MIMO-STS is added tothe third MIMO-LTS. The accurate channel can be estimated by correctingat the receiving side the thus reduced signal strength of the MIMO-LTS.

In the second modification, the same combination as in the firstmodification is also possible. It is effective to reduce the amplitudeof part of MIMO-LTS in FIG. 3B, FIGS. 19A and 19B and FIG. 21A to FIG.21D. The second modification can also be applied to the case of FIG.26A. More specifically, a steering matrix may be applied to the secondmodification. For instance, the second modification is applied to theburst format shown in FIG. 19A and thereafter a steering matrix isapplied to this burst format, which will result in the burst format asshown in FIG. 27.

Next, a third modification will be described. Similarly to the secondmodification, the third modification relates to MIMO-LTSs transmittedfrom antennas 12, among LTS antennas 12, other than Data antennas 12when the number of LTS antennas 12 is greater than the number of Dataantenna 12. Problems associated with this third modification areexpressed the same way as those in the first modification. In the caseof FIG. 19A, the antennas 12, among the LTS antenna 12, other than theData antennas 12 correspond to the third antenna 12 c. In the thirdmodification, the number of subcarriers used in the third MIMO-LTStransmitted from the third antenna 12 c is defined to be a value lessthan the number of subcarriers used in the first MIMO-LTS and secondMIMO-LTS transmitted from the first antenna 12 a and second antenna 12b.

This is equivalent to the case where “52 subcarries” are used in thefirst MIMO-LTS and second MIMO-LTS but “26 subacarriers” are used in thethird MIMO-LTS. The first antenna 12 a and second antenna 12 bcorrespond to Data antennas 12. According to the third modification, thesignal strength of the third MIMO-LTS can be made smaller by reducingthe number of subcarriers used for the third MIMO-LTS. When the signalstrength of the third MIMO-LTS gets small, the distortion againstMIMO-LTS becomes less likely to occur even if no MIMO-STS is added tothe third MIMO-LTS. Although part of the subcarriers is not used at atransmitting side, channels for all of the subcarriers can be estimatedif channels estimated for predetermined subcarriers are interpolated andsupplemented at a receiving side.

In the third modification, the same combination as in the firstmodification is also possible. In other words, it is effective not touse part of subcarriers in FIG. 3B, FIGS. 19A and 19B and FIGS. 21A to21D. The third modification can be applied to the case of FIG. 26A. Morespecifically, a steering matrix may be applied to the thirdmodification. For instance, the third modification is applied to theburst format shown in FIG. 19A and thereafter a steering matrix isapplied to this burst format, which will result in the burst format asshown in FIG. 27.

According to the present embodiment, when antennas that should transmitMIMO-STSs and Data are decided, an antenna whose strength of a signalreceived is larger is preferentially used. As a result, when a targetedradio apparatus receives a burst signal, the signal strength of MIMO-STSbecomes large to a certain degree. Hence, AGC is set in a manner thatthe gain is somehow low. As a result thereof, the occurrence probabilityof distortion against MIMO-LTS on account of AGC can be reduced. Sincethe occurrence probability of distortion against MIMO-LTS can bereduced, the deterioration of data error can be prevented. Since thedeterioration of data error can be prevented, the deterioration ofcommunication quality can be prevented. The channel estimation can bedone accurately. The data transmission efficiency can be improved.

The amplitude of the MIMO-LTSs transmitted from antennas other than Dataantennas is set to a value smaller than the amplitude of the otherMIMO-LTSs, so that the occurrence probability of distortion againstMIMO-LTS can be reduced at a receiving side. The number of subcarriersin MIMO-LTS transmitted from the antennas other than the Data antennasis set to a value smaller than the number of subcarriers in the otherMIMO-LTSs, so that the occurrence probability of distortion againstMIMO-LTS can be reduced at the receiving side. The third modificationcan be applied to the case when a steering matrix is used.

The present invention has been described based on the embodiments whichare only exemplary. It is therefore understood by those skilled in theart that other various modifications to the combination of eachcomponent and process described above are possible and that suchmodifications are also within the scope of the present invention.

In the first embodiment of the prevent invention, the selector 28selects preferentially the antennas 12 whose strengths of receivedsignals are larger. However, the present invention is not limitedthereto and, for example, delay spread may be derived for each of theantennas 12 and antennas 12 whose delay spreads are smaller may bepreferentially selected. According to this modification, the antennas 12having less effect of delayed wave can be selected. That is, thismodification can be used so long as the wireless quality of antennas 12is satisfactory.

In the first embodiment of the present invention, the first radioapparatus 10 a controls so that the number of antennas 12 to be usedwhen the training signals are transmitted is equal to the number ofantennas 12 to be used when the training signals are received. However,the present invention is not limited thereto and different controls maybe exercised. For example, the processing unit 22 receives trainingsignals for use with receiving, from the second radio apparatus 10 b viaa plurality of antennas 12, and the selector 28 selects at least oneantenna, among a plurality of antennas, that should transmit thetraining signals. In this case, the selector 28 may derive wirelessqualities corresponding respectively to a plurality of antennas 12,based on the received training signals for use with receiving and thenselect preferentially antennas whose wireless qualities are desirable.According to this modification, the number of transmitting antennas 12and the number of receiving antennas 12 can be set independently fromeach other.

A modification resulting from the combination of the first embodimentand the second embodiment is also effective. For instance, in thepresent embodiments, the number of a plurality of streams finallytransmitted from the radio unit 20 may be less than the number ofantennas 12, in accordance with description of the first embodiment.According to this modification, the advantageous aspects gained bycombining the first embodiment with the second embodiment are obtained.

Arbitrary combination among the first to fourth embodiments is alsoeffective. According to this modification, the advantageous aspectsgained by this combination thereamong is obtained.

Data may be composed of a plurality of streams. A known signal may becomposed of a plurality of streams. A control signal may be composed ofa plurality of streams.

It is to be noted that any arbitrary combination of the above-describedstructural components and expressions changed among a method, anapparatus, a system, a recording medium, a computer program and so forthare all effective as and encompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be sub-combinationof these described features.

While the preferred embodiments of the present invention have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the appendedclaims.

1-19. (canceled)
 20. A radio apparatus, comprising: a generating unitconfigured for generating a burst signal that comprises (1) first knownsignal(s) operative in a MIMO system assigned to N of M streams (N<M),(2) second known signals operative in the MIMO system assigned to therespective M streams, and (3) data assigned to the N of M streams, the Nof M streams including the first known signal, the second known signal,and the data in that order; and a plurality of antennas configured fortransmitting the burst signal generated by the generating unit.
 21. Theradio apparatus according to claim 20, wherein N is variable.
 22. Aradio apparatus, comprising: a generating unit configured for generatinga burst signal that comprises (1) first known signal(s) operative in aMIMO system assigned to N of M streams (N<M), (2) second known signalsoperative in the MIMO system assigned to the respective M streams, and(3) data assigned to the N of M streams, the N of M streams includingthe first known signal, the second known signal, and the data in thatorder; a power amplifier configured for amplifying the burst signalgenerated by the generating unit; and a plurality of antennas configuredfor transmitting the burst signal amplified by the power amplifier. 23.The radio apparatus according to claim 22, wherein N is variable.
 24. Aradio apparatus, comprising: a generating unit configured for generatinga digital burst signal that comprises (1) first known signal(s)operative in a MIMO system assigned to N of M streams (N<M), (2) secondknown signals operative in the MIMO system assigned to the respective Mstreams, and (3) data assigned to the N of M streams, the N of M streamsincluding the first known signal, the second known signal, and the datain that order; a D-A conversion unit configured for converting thedigital burst signal generated by the generating unit into an analogburst signal; a power amplifier configured for amplifying the analogburst signal converted by the D-A conversion unit; and a plurality ofantennas configured for transmitting the analog burst signal amplifiedby the power amplifier.
 25. The radio apparatus according to claim 24,wherein N is variable.
 26. A radio apparatus, comprising; a generatingunit configured for generating a burst signal that comprises (1) firstknown signal(s) operative in a MIMO system assigned to N of M streams(N<M), (2) second known signals operative in the MIMO system assigned tothe respective M streams, and (3) data assigned to the N of M streams,the N of M streams including the first known signal, the second knownsignal, and the data in that order; and a power amplifier configured foramplifying the burst signal generated by the generating unit.
 27. Theradio apparatus according to claim 26, wherein N is variable.
 28. Aradio apparatus, comprising: a generating unit configured for generatinga digital burst signal that comprises (1) first known signal(s)operative in a MIMD system assigned to N of M streams (N<M), (2) secondknown signals operative in the MIMO system assigned to the respective Mstreams, and (3) data assigned to the N of M streams, the N of M streamsincluding the first known signal, the second known signal, and the datain that order; a D-A conversion unit configured for converting thedigital burst signal generated by the generating unit into an analogburst signal; and a power amplifier configured for amplifying the analogburst signal converted by the D-A conversion unit.
 29. The radioapparatus according to claim 28, wherein N is variable.